Setting up your environment for working on the Pharo VM

To work on Pharo’s virtual machine, you’ll need to set up both the virtual machine you’ll be modifying, as well as a Pharo image. Below are the instructions required to set up the environment. You will need both the VM and the image containing the VM compilation chain (VMMaker package).

Building the Pharo VM

  • Clone the pharo-vm repo ( .
    • You may need to switch to a branch different from the default one, depending on top of which version you want to make your changes.
  • Check you have all dependencies needed to build the pharo-vm
    • Tip: When working on macOs, you can install Command Line Tools for Xcode, that already contains many of the dependencies you’ll need.
  • Build the virtual machine to check everything went correctly:
    • To do so, run the following command inside pharo-vm‘s root directory: cmake --build . --target install
    • If compilation completes without error, you’re good to go!

Configuring the Pharo Image with VMMaker

  • Download PharoLauncher from
  • Create a new Pharo image (if in doubt, choose the latest stable version)
  • Set-up pharo-vm in Pharo
    • Open Pharo image
    • Add the pharo-vm github repository to the Pharo image via Iceberg (Add > Import from existing clone)
    • Right-click repo -> Metacello > Install baseline of VMMaker (Default)
  • Verify that the installation completed correctly
    • Run all VMMakerTests test suites
    • If all tests pass (except from the JIT-related ones), you are good to go!
  • To run a Pharo image with a specific version of the VM, you can use the following command in the root of pharo-vm‘s root directory:
    • build % ./build/dist/ ~/Documents/Pharo/images/Pharo\ 10.0\ -\ 64bit\ \(stable\)/Pharo\ 10.0\ -\ 64bit\ \(stable\).image --interactive
    • Note: the exact path may differ in your case, depending on your Pharo version and OS.

Now you are ready to go and change the VM code and generate a new image.

What is RBParseTreeSearcher ?

Imagine that you want to find a specific expression and that you want to find it in the complete system. How many classes would you have to look for? How can you be sure that you did not miss any class and being sure that you won’t be frustrated because of the number of issues thrown on compilation or execution? In addition imagine other scenario where you want to transform that expression into another one.

Changing code, removing, or replacing deprecated methods is costly for a developer by doing it manually instead of using an automated feature.

This blog post will explain how to find a specific piece of code we may look for inside a Pharo program, and make it easy for the developers to deal with pattern matching and RBParseTreeSearcher class.

Following work will be about how to replace code using RBParseTreeRewriter and doing the exact same thing automatically using the Rewrite tool (a tool built on top the RBParseTreeRewriter). 

For the moment, we will explain some fundamental definitions and for that the post is structured following below sections:

  1. Pattern code description
  2. RBParseTreeSearcher description
  3. RBParseTreeSearcher examples with pattern code

1. Pattern code description

A pattern expression is very similar to an ordinary Smalltalk expression, but allows one to specify some “wildcards”. The purpose is simple. Imagine that you have a piece of code:

car isNil ifTrue: [ ^ self ].

You can of course compare it with the same piece of code for equality, but wouldn’t it be cool if you could compare something similar, but ignore the fact that the receiver is named car? With pattern rules you can do exactly that. Consider the following code and notice the back-tick before car:

`car isNil ifTrue: [ ^ self ].

Now this expression can match any other expression where isNil ifTrue: [^self] is sent to any variable (or literal). With such a power you can find all the usages of isNil ifTrue: and replace them with ifNil. So what are the “wildcards” that we are using?

(`)Basic pattern nodes

Code prefixed with a back-tick character (`) defines a pattern node. The table below is listing three simple patterns that can be declared with the back-tick:

Pattern typeExampleDescription
Variable`someName asStringThis pattern will match message asString sent to any receiver, disregarding the name of it
MessagePharo globals `someMessageThis pattern will match any unary message sent to Pharo globals.
Method`someMethod ^ nilThis pattern will match any method which returns nil
Selector`sel: aValThis pattern will match any selector followed by aVal.

Example with matches:

`receiver foo 


  • self foo
  • x foo
  • OrderedCollection foo
(`#) Literal pattern nodes

A back-tick can be followed by the hash sign to ensure that matched receiver will be a literal:

Pattern typePattern nodeDescription
Literal`#literal asArrayThis pattern will match any literal (Number, String, Array of literal ) followed by asArray message


 `#lit size


  • 3 size
  • 'foo' size
  • #(a b c) size
(`@) List pattern nodes

To have complete flexibility, there is the possibility to use an at sign @ before the name of a pattern node which turns the node into a list pattern node, which can be empty, returns one or multiple values.

Pattern typePattern nodeDescription
Entity`@expr isPatternVariableThis pattern will match a single or multiple entities followed by isPatternVariable
MessagemyVar `@messageThis pattern will match any message (including unary) sent to myVar
Temporary variable|`temp `@temps|This pattern will match at least one temporary variable which is defined as `temp; For`@temps, the matching can find nil, one or many temporary variables defined.
ArgumentmyDict at: 1 put:`@argsThis pattern will match myDict at: 1 put: followed by a list of arguments `@args that can be nil, one or many args.
List of statements[ `.@statements.
 `var := `myAttribute. ]
We will explain statements later on, but this is to mention that @ can be used also to define a list of statements which can be empty, contain one or many elements.

This expression will match a block which has at first a list of statements, that must be followed by 1 last assignment statement `var := `myAttribute.


  • You may write an expression with just args instead of `@args.
  • The list patterns does not make any sense for literal nodes i.e. `#@literal.

Example 1:

`x := `@value


myVar := OrderedCollection new

Example 2:

`sel1 at: `@args1 `sel2: `@args2


self at: index putLink: (self linkOf: anObject ifAbsent: [anObject asLink])


  • `args1 and `args2 have different values
  • `sel1 matches self
  • `@args1 matches index
  • `sel2: matches putLink:
  • `@args2 matches (self linkOf: anObject ifAbsent: [anObject asLink])

Example 3:

`@rcvr `@msg: `@args matches:

(self class deferUpdates: true) ifTrue: [^aBlock value].


  • `@rcvr matches (self class deferUpdates: true)
  • `@msg: matches ifTrue:
  • `@args matches [^aBlock value]

Example 4:

|`@args1 `myArgument `@args2| matches:

| t1 t2 |

Here we need to have at least 1 argument myArgument , and the example is matching because `@args1 can be empty. So here we have:

  • myArgument is matching with t1
  • `@args2 is matching with t2
(`.) Statement pattern nodes

Back-tick can be followed by a period to match statements. For example:

Pattern typePattern nodeDescription
ifTrue: [`.statement1 ]
ifFalse: [ `.statement2 ]
This pattern will match an ifTrue:ifFalse: message send to any variable, where both blocks have only one statement each.



is matching:

  • x := 1.
  • myVal:= 'Hello World'.
  • self assert: myVal size equals: 11.




x := 1.
x := 2


  • |`@temps| matches |x|
  • `@.statements1. is nil
  • `.duplicate. matches x := 1.
  • `@.statements2

P.S. In the end it does not matter whether you will write `.@Statement or `@.Statement.

(`{ }) Block Pattern Nodes

These are the most exotic of all the nodes. They match any AST nodes like a list pattern and test it with a block. The syntax is similar to the Pharo block, but curly braces are used instead of square brackets and as always the whole expression begins with a back-tick.

Consider the following example:

`{ :node | node isVariable and: [ node isGlobal ] } become: nil

this pattern will match a message #become: with an attribute nil, where the receiver is a variable and it is a global variable. 

There is also a special case called wrapped block pattern node which has the same syntax and follows a normal pattern node. In this case first the node will be matched based on its pattern, and then passed to the block. For example:

`#arr `{ :node | node isLiteralArray } asArray

is a simple way to detect expression like #(1 2 3) asArray. In this case first #(1 2 3) will be matched by the node and then tested by the block.

Naming is Important

The pattern nodes are so that you can match anything in their place. But their naming is also important as the code gets mapped to them by name. For example:

`block value: `@expression value: `@expression

will match only those #value:value: messages that have exactly the same expressions as both arguments. It is like that because we used the same pattern variable name.

2. RBParseTreeSearcher description

So, after figuring out what are the patterns that can be used and what kind of matches they can perform, now we can move forward to discover how RBParseTreeSearcher class works in Pharo , in order to be able to understand in the last section how RBParseTreeSearcher and defined patterns work together to find the matches we are looking for.

RBParseTreeSearcher is supposed to look for a defined pattern using the ‘wildcards’ of a matcher defined as a Tree, and on success (when match is found) a block can be executed.

Basically, when a developper uses this class, the most used instance variables are:

  • #matches:do: which a message that looks for patterns defined in matches: block using the wildcards, and if a match is found the do: block is executed.
    The do block takes two parameters: :aNode and :answer. The aNode refers to each node of the pattern defined, and the answer can be used for example to increment value on each node match.
    The blocks defined in #matches:do: are called rules, and they are stored only in success in instance searches of RBParseTreeSearcher defined below.
  • searches which type is Ordered collection, contains all the successful rules applied whenever using: #matches:do:, #matchesMethod:do … to store rules of type Rule, MethodRule, ArgumentRule, TreeRule …
  • context which type is dictionary: contains all the successfully matched patterns.
  • executeTree: this method takes aParseTree as input parameter, which is the possible matching code that we are looking for, and starts the matching process using the defined pattern.
  • messages of type OrderedCollection, and returns the list of messages found in a match.
  • hasRules returns searches list

Consider the following example which is using the instance sides defined above:

|searcher dict|
searcher := RBParseTreeSearcher new.
    matches: '`@rcv at:`@arg `sel:`@arg1'
    do: [ :aNode :answer | dict := searcher context ].
searcher executeTree:
    (RBParser parseExpression: 'cache at: each ifAbsentPut: [ each ].').

The method #matches:do: is used to define the pattern that we are looking for, using the ‘wildcards’ defined in first section; In addition of that, the do is running only on match, and in our case it will fill the dictionary dict with the searcher context (which is the pattern defined in matches block).
This execution is fired on executeTree: which defines the matcher that is a String parsed as a Tree using parseExpression, then starts matching it with the pattern.

3. RBParseTreeSearcher examples with pattern code

Finally, in this section we use patterns with the RBParseTreeSearcher class and do some magic by finding some matches in Pharo code !

Consider the following example:

| dict searcher|
searcher := RBParseTreeSearcher new.

   matches: '`@receiver assert: `@arg equals: true'
   do: [ :aNode :answer | dict := searcher context ].

   executeTree: (RBParser parseExpression: 'self assert: reader storedSettings first realValue equals: true.').

   collect: [ :each | each displayString ].

The example is matching successfully and the dictionary dict will return different values during the iteration:

Match 1: (key) `@receiver is matching with (value) self
Match 2: (key) `@arg is matching with (value) reader storedSettings first realValue

If we want to check all the messages in the matcher, we can use searcher messages which will return an array of one item containing message #assert:equals: as it is the only message available in the matched expression.


Using styles in Spec applications

In this post we will see how to use custom styles in Spec applications. We will start to present styles and then build a little editor as the one displayed hereafter.

We will show that an application in Spec manages styles and let you adapt the look of a presenter.

How do styles work?

Styles in Spec work like CSS. They are style sheets in which the properties for presenting a presenter are defined. Properties such as colors, width, height, font, and others. As a general principle it is better to use styles instead of fixed constraints, because your application will be more responsive.

For example, you want a button to have a specific width and height. You can do it using constraints with the method add:withConstraints: or using styles. In both cases the result will be this:

But, if you change the size of the fonts of the Pharo image using Settings/Appearance/Standard Fonts/Huge, using fixed constraints, you will obtain the following result. You will for example do not be able to see the icons because the size is not recomputed correctly.

If you use styles, the size of the button will also scale as shown below.

Style format

The styles in Spec format are similar to CSS. Style style sheets are written using STON as format. We need to write the styles as a string and then parse it as a STON file.

Here is an example that we will explain steps by steps below.

'.application [       
    .lightGreen [ Draw { #color: #B3E6B5 } ],          
    .lightBlue [ Draw { #color: #lightBlue } ] ]'

We will go by steps.

SpPropertyStyle has 5 subclasses: SpContainerStyle, SpDrawStyle, SpFontStyle, SpTextStyle, and SpGeometryStyle. These subclasses define the 5 types of properties that exist. On the class side, the method stonName that indicates the name that we must put in the STON file.

  • SpDrawStyle modifies the properties related to the drawing of the presenter, such as the color and the background color.
  • SpFontStyle manipulates all related to fonts.
  • SpGeometryStyle is for sizes, like width, height, minimum height, etc.
  • SpContainerStyle is for the alignment of the presenters, usually with property is changed on the main presenter, which is the one that contains and arranges the other ones.
  • SpTextStyle controls the properties of the SpTextInputFieldPresenter.

If we want to change the color of a presenter, we need to create a string and use the SpDrawStyle property, which STON name is Draw as shown below. For setting the color, we can use either the hexadecimal code of the color or the sender of Color class.

'.application [       
    .lightGreen [ Draw { #color: #B3E6B5 } ],          
    .lightBlue [ Draw { #color: #lightBlue } ] ]'

Now we have two styles: lightGreen and lightBlue that can be applied to any presenter.

We can also use environmental variables to get the values of the predefined colors of the current theme, or the fonts. For example, we can create two styles for changing the fonts of the letters of a presenter:

'.application [
    .codeFont [ Font { #name: EnvironmentFont(#code) } ],
    .textFont [ Font { #name: EnvironmentFont(#default) } ]

Also we can change the styles for all the presenters by default. We can put by default all the letters in bold.

'.application [
	Font { #bold: true }

Defining an Application

To use styles we need to associate the main presenter with an application. The class SpApplication already has default styles. To not redefine all the properties for all the presenters, we can concatenate the default styles (SpStyle defaultStyleSheet) with our own. As said above, the styles are actually STON files that need to be parsed. To parse the string into a STON we can use the class SpStyleVariableSTONReader.

presenter := SpPresenter new.
presenter application: (app := SpApplication new).

styleSheet := SpStyle defaultStyleSheet, 
	(SpStyleVariableSTONReader fromString: 
	'.application [
	     Font { #bold: true },
            .lightGreen [ Draw { #color: #B3E6B5 } ],
            .bgBlack [ Draw { #backgroundColor: #black } ],
	    .blue [ Draw { #color: #blue } ]
]' ).

app styleSheet: styleSheet.

Now, can can add one or more styles to a presenter, like follows:

presenter layout: (SpBoxLayout newTopToBottom
	add: (label := presenter newLabel);

label label: 'I am a label'.
label addStyle: 'lightGreen'.
label addStyle: 'black'.
presenter openWithSpec.

Also we can remove and add styles at runtime.

label removeStyle: 'lightGreen'.
label removeStyle: 'bgBlack'.
label addStyle: 'blue'.

Using classes

To properly use styles, it is better to define a custom application as a subclass of SpApplication.

SpApplication << #CustomStylesApplication
	slots: {};
	package: 'Spec-workshop'

In the class we need to override the method styleSheet to return our custom style sheet concatenated with the default one.

CustomStylesApplication >> styleSheet

	^ SpStyle defaultStyleSheet, 
	(SpStyleVariableSTONReader fromString:
'.application [
	Font { #bold: true },

	.lightGreen [ Draw { #color: #B3E6B5 } ],
	.lightBlue [ Draw { #color: #lightBlue } ],
	.container [ Container { #padding: 4, #borderWidth: 2 } ],
	.bgOpaque [ Draw { #backgroundColor: EnvironmentColor(#base) } ],
	.codeFont [ Font { #name: EnvironmentFont(#code) } ],
	.textFont [ Font { #name: EnvironmentFont(#default) } ],
	.bigFontSize [ Font { #size: 20 } ],
	.smallFontSize [ Font { #size: 14 } ],
	.icon [ Geometry { #width: 30 } ],
	.buttonStyle [ Geometry { #width: 110 } ],
	.labelStyle [ 
		Geometry { #height: 25 },
		Font { #size: 12 }	]

We can use different properties in the same style. For example, in labelStyle we are setting the height of the presenter to 25 scaled pixels and the font size to 12 scaled pixels. Also, we are using EnvironmentColor(#base)for obtaining the default background colour according to the current theme. Because the colour will change according to the theme that used in the image.

For the main presenter, we will build a mini-text-viewer in which we will be able to change the size and the font of the text that we are viewing.

SpPresenter << #CustomStylesPresenter
	slots: { #text . #label . #zoomOutButton . #textFontButton . #codeFontButton . #zoomInButton };
	package: 'Spec-workshop'

In the initializePresenters method we will first initialise the presenters, then set the styles for the presenters and finally initialise the layout.

CustomStylesPresenter >> initializePresenters

	self instantiatePresenters.
	self initializeStyles.
	self initializeLayout
CustomStylesPresenter >> instantiatePresenters

	zoomInButton := self newButton.
	zoomInButton icon: (self iconNamed: #glamorousZoomIn).
	zoomOutButton := self newButton.
	zoomOutButton icon: (self iconNamed: #glamorousZoomOut).

	codeFontButton := self newButton.
		icon: (self iconNamed: #smallObjects);
		label: 'Code font'.
	textFontButton := self newButton.
		icon: (self iconNamed: #smallFonts);
		label: 'Text font'.

	text := self newText.
		text: String loremIpsum.

	label := self newLabel.
	label label: 'Lorem ipsum'
CustomStylesPresenter >> initializeLayout
	self layout: (SpBoxLayout newTopToBottom
		add: label expand: false;
		add: (SpBoxLayout newLeftToRight
			add: textFontButton expand: false;
			add: codeFontButton expand: false;
			addLast: zoomOutButton expand: false;		
			addLast: zoomInButton expand: false;
		expand: false;
		add: text;

Finally, we change the window title and size:

CustomStylesPresenter>> initializeWindow: aWindowPresenter

		title: 'Using styles';
		initialExtent: 600 @ 400

Without setting the custom styles nor using our custom application in the presenter, we have:

We do not want the black background color for the text presenter. We will like to have a sort of muti-line label. We want the zoom buton to be smaller as they only have icons. We want to have the option to change the size and font of the text inside the text presenter. Finally, why not, we want to change the color of the label, change the height and make it a little more bigger.

CustomStylesPresenter >> initializeStyles

    "Change the height and size of the label."
    label addStyle: 'labelStyle'.
    "But the color as light green"
    label addStyle: 'lightGreen'.

    "The default font of the text will be the code font and the size size will be the small one."
    text addStyle: 'codeFont'.
    text addStyle: 'smallFontSize'.
    "Change the background color."
    text addStyle: 'bgOpaque'.

    "But a smaller width for the zoom buttons"
    zoomInButton addStyle: 'icon'.
    zoomOutButton addStyle: 'icon'.
    codeFontButton addStyle: 'buttonStyle'.
    textFontButton addStyle: 'buttonStyle'.

    "As this presenter is the container, set to self the container
    style to add a padding and border width."
    self addStyle: 'container'

Finally, we have to override the start method in the application. With this, we are going to set the application of the presenter and run the presenter from the application.

CustomStylesApplication >> start

	(self new: CustomStylesPresenter) openWithSpec

Now, if we run CustomStylesApplication new start we will have:

The only thing missing is to add the behaviour when pressing the buttons.

For example, if we click on the zoom in button we want to remove the smallFontStyle and add the bigFontSize. Or, if we click on the text font button, we want to remove the style codeFont and add the textFont style. So, in the connectPresenters method we have:

CustomStylesPresenter >> connectPresenters

	zoomInButton action: [
		text removeStyle: 'smallFontSize'.
		text addStyle: 'bigFontSize' ].
	zoomOutButton action: [ 
		text removeStyle: 'bigFontSize'.
		text addStyle: 'smallFontSize'].

	codeFontButton action: [
		text removeStyle: 'textFont'.
		text addStyle: 'codeFont' ].
	textFontButton action: [ 
		text removeStyle: 'codeFont'.
		text addStyle: 'textFont']

Now, if we click on zoom in we will have:

And if we click on text font:


Using styles in Spec is great. It make easier to have a consistent design as we can add the same style to several presenters. If we want to change some style, we only edit the styles sheet. Also, the styles automatically scale if we change the font size of all the image. They are one of the main reason why in Spec we have the notion of an application. We can dynamically change how a presenter looks.

Sebastian Jordan-Montano

Binding an external library into Pharo

In this post I am going to show you how to call external functions from a Pharo image. Here, we are going to use the LAPACK (Linear Algebra Package) library that is written in Fortran.

Why do we need this library?

In the Pharo AI project (, we are working on an implementation of linear regression. Currently, we are writing the logic completely in Pharo. But, linear regression can be formulated in terms of least squares and Lapack implements efficiently the minimum-norm solution to a linear least squares problem.

So we want to get the best of the two worlds: nice objects in Pharo and call some fast and low-level libraries for crunching numbers. We want to bind the routine dgelsd() that does exactly what we want.

Implementing the binding

We need to have the library already installed in our machines. As a first step, we create a subclass of FFILibrary called LapackLibrary and we need to override the methods: macLibraryName, win32LibraryName and unixLibraryName. In those methods we should return the path in which the library is installed. For MacOS, we override the macLibraryName method as follows:

LapackLibrary >> macLibraryName
	^ FFIMacLibraryFinder new 
		userPaths: { '/usr/local/opt/lapack/lib/' };
		findAnyLibrary: #('liblapack.dylib').

And for Windows:

LapackLibrary >> win32LibraryName

	^ FFIWindowsLibraryFinder new
		userPaths: { '/usr/local/opt/lapack/lib/' };
		findAnyLibrary: #( 'liblapack.dylib' )

For using this binding on Linux is only needed to override the remaining method. One can use the class FFIUnix32LibraryFinder or FFIUnix64LibraryFinder.

Now, we are going to create the class LapackDgelsd. We override the method ffiLibrary to return just the class LapackLibrary.

LapackDgelsd >> ffiLibrary

	^ LapackLibrary

Now we can implement the method which will eventually make the FFI call. We saw in the documentation that dgelsd receives 14 parameters, all pointers, that have different types. To make the FFI call we have to use self ffiCall: and inside put the signature of the foreign function.

The variables that are passed must be either local or instance variables. We can specify the type of each of the variables, or we can say that the type is void*. That means that the FFI library is not going to do the mapping to the correct type, but is a responsiblity of the programmer to instantiate the variables with the correct type. So, we get:

LapackDgelsd >> ffiDgelsdM: m n: n nrhs: nrhs a: a lda: lda b: b ldb: ldb s: s rcond: rcond rank: aRank work: work lwork: lwork iwork: iwork info: anInfo
	^ self ffiCall: #( void dgelsd_(
		void* m,
        void* n,
        void* nrhs,
    	void* a,
        void* lda,
    	void* b,
        void* ldb,
    	void* s,
    	void* rcond,
        void* aRank,
    	void* work,
        void* lwork,
    	void* iwork,
    	void* anInfo ) )

In the documentation of the routine , we see that we need integer pointers, double pointers, integer arrays, and double arrays. To use pointers in Pharo, we need to use the class FFIExternalValueHolder. It actually will create an anonymous class of the type that we need. To ease the work, we will create a helper class that will create the pointer for us.

We can name the class LapackPointerCreator and that class has to have two class variables: DoublePointerClass and IntegerPointerClass. In the initialize method of the class, we instantiate the value of the class variables to be:

LapackPointerCreator class >> initialize
	DoublePointerClass := FFIExternalValueHolder ofType: 'double'.
	IntegerPointerClass := FFIExternalValueHolder ofType: 'int'

And then we create the two helper methods:

LapackPointerCreator class >> doublePointer: aNumber

	^ DoublePointerClass new value: aNumber
LapackPointerCreator class >> integerPointer: aNumber

	^ IntegerPointerClass new value: aNumber

And we will also create an extension method of Collection to convert a collection into n FFI external array.

Collection >> asFFIExternalArrayOfType: aType
	"Converts collection to FFIExternalArray.
	#(1 2 3) asFFIExternalArrayOfType: 'int'"
	| array |
	array := FFIExternalArray newType: aType size: self size.
	self withIndexDo: [ :each :index | array at: index put: each ].
	^ array

Now we are ready to make the call to the Lapack dgelsd() routine. We are going to follow this example of how to use dgelsd() with the same values. First, we need to create all the variables that we are going to pass as arguments.

numberOfRows := 4.
numberOfColumns := 5.
numberOfRightHandSides := 3.

matrixA := #( 0.12 -6.91 -3.33  3.97 -8.19
			  2.22 -8.94  3.33  7.69 -5.12
			 -6.72 -2.74 -2.26 -9.08 -4.40
		 	 -7.92 -4.71  9.96 -9.98 -3.20 ) asFFIExternalArrayOfType: 'double'.

matrixB := #( 7.30  1.33  2.68 -9.62 0.00
		  	  0.47  6.58 -1.71 -0.79 0.00
			 -6.28 -3.42  3.46  0.41 0.00 ) asFFIExternalArrayOfType: 'double'.

reciprocalConditionNumberPointer := LapackPointerCreator doublePointer: -1.

singularValuesArray := FFIExternalArray newType: 'double' size: numberOfRows.
iworkArray := FFIExternalArray newType: 'int' size: 11 * numberOfRows.

numberOfRowsPointer := LapackPointerCreator integerPointer: numberOfRows.
numberOfColumnsPointer := LapackPointerCreator integerPointer: numberOfColumns.

numberOfRightHandSidesPointer := LapackPointerCreator integerPointer:

leadingDimensionAPointer := LapackPointerCreator integerPointer: numberOfRows.
leadingDimensionBPointer := LapackPointerCreator integerPointer: numberOfColumns.

rankPointer := LapackPointerCreator integerPointer: 0.
infoPointer := LapackPointerCreator integerPointer: 0.

The WORK array is a workspace that is used internally by the dgelsd() routine. We must allocate the memory for this array before passing it to the routine. To calculate the optimal size of the WORK array, we run the dgelsd() routine with LWORK value -1 and WORK as any pointer. After this first execution, the optimal workspace size will be written into the WORK pointer. Therefore, we will be making two FFI calls to the routine: first to calculate the optimal workspace and then to find the solution to the least squares problem.

lworkPointer := LapackPointerCreator integerPointer: -1.
workPointer := LapackPointerCreator doublePointer: 0.
dgelsd := LapackDgelsd new.

	ffiDgelsdM: numberOfRowsPointer 
	n: numberOfColumnsPointer 
	nrhs: numberOfRightHandSidesPointer
	a: matrixA 
	lda: leadingDimensionAPointer 
	b: matrixB 
	ldb: leadingDimensionBPointer 
	s: singularValuesArray
	rcond: reciprocalConditionNumberPointer 
	rank: rankPointer
	work: workPointer 
	lwork: lworkPointer
	iwork: iworkArray 
	info: infoPointer.

Now, the variable workPointer contains the value of the optimal workspace. With that information, we run the dgelsd() routine again to solve the problem for matrices A and B.

optimalWorkspace := workPointer value asInteger.
workArray := FFIExternalArray newType: 'double' size: optimalWorkspace.
workArraySizePointer := LapackPointerCreator integerPointer: optimalWorkspace.

	ffiDgelsdM: numberOfRowsPointer 
	n: numberOfColumnsPointer 
	nrhs: numberOfRightHandSidesPointer
	a: matrixA 
	lda: leadingDimensionAPointer 
	b: matrixB 
	ldb: leadingDimensionBPointer 
	s: singularValuesArray
	rcond: reciprocalConditionNumberPointer 
	rank: rankPointer
	work: workArray 
	lwork: workArraySizePointer 
	iwork: iworkArray 
	info: infoPointer

The result of the computation is stored in several variables. For example, the values of matrixB have been replaced with the minimum norm solution. The effective rank is contained in the rankPointer and the S array contains the singular values of matrix A in decreasing order. The INFO variable contains an integer value that informs us whether the routine succeded or failed (0 = successful exit).

To see the value of a pointer, we can use the value message, for example, rankPointer value.

Improving the API

Make the call to the Fortran routine was a little tricky. We don’t want to give the user the responsibility of creating all of those pointers for using the not-so-nice method signature. So we will use Pharo in our favour.

Actually, we only need to set 5 values of the method, the rest can be calculated internally. What we will do is to have all the needed variables as instance variables of the class LapackDgelsd. Like:'matrixA matrixB numberOfRows numberOfColumns numberOfRightHandSides leadingDimensionA leadingDimensionB singularValues rank info reciprocalConditionNumber workArraySize minimumNormSolution iworkArray'.

Then, we will create the setters for the 5 values that we need the user to insert. Note that the user does not need to create any pointer. They pass only a Pharo Array.

LapackDgelsd >> numberOfColumns: aNumber

	numberOfColumns := aNumber
LapackDgelsd >> numberOfRows: aNumber

	numberOfRows := aNumber
LapackDgelsd >> matrixA: anArray

	matrixA := anArray asFFIExternalArrayOfType: 'double'
LapackDgelsd >> matrixB: anArray

	matrixB := anArray asFFIExternalArrayOfType: 'double'
LapackDgelsd >> numberOfRightHandSides: aNumber

	numberOfRightHandSides := aNumber

Now, as we do not want to give the responsiblity to the user to call the method twice, one for obtaining the optimal workspace and then for solving the actual equation. We will create a solving method that does all the work. We have:

LapackDgelsd >> solve

	| singularValuesArray numberOfRowsPointer numberOfColumnsPointer
      numberOfRightHandSidesPointer leadingDimensionAPointer leadingDimensionBPointer
      rankPointer infoPointer reciprocalConditionNumberPointer workArray
      workArraySizePointer |

	singularValuesArray := FFIExternalArray newType: 'double' size: numberOfRows.
	"iwork dimension should be at least 3*min(m,n)*nlvl + 11*min(m,n),
     where nlvl = max( 0, int( log_2( min(m,n)/(smlsiz+1) ) )+1 )
     and smlsiz = 25"
	iworkArray := FFIExternalArray newType: 'int' size: 11 * numberOfRows.
	numberOfRowsPointer := LapackPointerCreator integerPointer: numberOfRows.
	numberOfColumnsPointer := LapackPointerCreator integerPointer: numberOfColumns.
	numberOfRightHandSidesPointer := LapackPointerCreator 
        integerPointer: numberOfRightHandSides.
	leadingDimensionAPointer := LapackPointerCreator integerPointer: self leadingDimensionA.
	leadingDimensionBPointer := LapackPointerCreator integerPointer: self leadingDimensionB.
	rankPointer := LapackPointerCreator integerPointer: 0.
	infoPointer := LapackPointerCreator integerPointer: 0.
	reciprocalConditionNumberPointer := LapackPointerCreator 
        doublePointer: self reciprocalConditionNumber. 
	workArraySize := self
		findOptimalWorkspace: numberOfRowsPointer 
		n: numberOfColumnsPointer 
		nrhs: numberOfRightHandSidesPointer
		a: matrixA 
		lda: leadingDimensionAPointer 
		b: matrixB 
		ldb: leadingDimensionBPointer 
		s: singularValuesArray
		rcond: reciprocalConditionNumberPointer 
		rank: rankPointer 
		work: nil  
		iwork: iworkArray 
		info: infoPointer.

	workArray := FFIExternalArray newType: 'double' size: workArraySize.
	workArraySizePointer := LapackPointerCreator integerPointer: workArraySize.

		ffiDgelsdM: numberOfRowsPointer 
		n: numberOfColumnsPointer 
		nrhs: numberOfRightHandSidesPointer
		a: matrixA 
		lda: leadingDimensionAPointer 
		b: matrixB 
		ldb: leadingDimensionBPointer 
		s: singularValuesArray
		rcond: reciprocalConditionNumberPointer 
		rank: rankPointer
		work: workArray 
		lwork: workArraySizePointer 
		iwork: iworkArray 
		info: infoPointer.

	minimumNormSolution := matrixB asArray.
	singularValues := singularValuesArray asArray.
	rank := rankPointer value.
	info := infoPointer value.
LapackDgelsd >> findOptimalWorkspace: m n: n nrhs: nrhs a: a lda: lda b: b ldb: ldb s: s rcond: rcond rank: aRank work: work iwork: iwork info: anInfo

	| lworkPtr workPtr |
	lworkPtr := LapackPointerCreator integerPointer:  -1.
	workPtr := LapackPointerCreator doublePointer:  0.

	self ffiDgelsdM: m n: n nrhs: nrhs a: a lda: lda b: b ldb: ldb s: s rcond: rcond rank: aRank work: workPtr lwork: lworkPtr iwork: iwork info: anInfo.
	^ workPtr value asInteger

Now, all that we have to do, as users, is:

numberOfRows := 4.
numberOfColumns := 5.
numberOfRightHandSides := 3.
matrixA := #( 0.12 -6.91 -3.33  3.97 -8.19
			  2.22 -8.94  3.33  7.69 -5.12
		     -6.72 -2.74 -2.26 -9.08 -4.40
		 	 -7.92 -4.71  9.96 -9.98 -3.20 ).
matrixB := #( 7.30  1.33  2.68 -9.62 0.00
		  	  0.47  6.58 -1.71 -0.79 0.00
		     -6.28 -3.42  3.46  0.41 0.00 ).

algorithm := LapackDgelsd new
    numberOfRows: numberOfRows;
    numberOfColumns: numberOfColumns;
    numberOfRightHandSides: numberOfRightHandSides;
    matrixA: matrixA;
    matrixB: matrixB;

algorithm solve.

This is definitively better, since it hides the implementation details of the underlying implementation. And for getting the solution, we only need to call these accessors:

"Info represents if the process completed with success"
algorithm info.

"The array with the solutions"
algorithm minimumNormSolution.

"The effective rank"
algorithm rank.

"And the singular values of matrix A"
algorithm singularValues.


As we saw, doing the binding for the first method was the hardest part. But all the next methods will be easier to implement because we can use the same infrastructure.

Here we showed a binding for a widely used linear algebra library. That will help us to speed up the mathematical computations of libraries such as PolyMath ( and Pharo-AI ( Lapack is a huge library so we don’t want to bind all the methods, but only the ones that we need. If people from the community would like us to migrate other methods of the library, we will be happy to do it. We will work on demand. The code and instructions are available here.

Our next step is to use dgelsd() in our linear regression implementation in Pharo for benchmarking against Python and R. Because those languages use also Lapack fortran library and they are considered as industry standards.

Sebastian Jordan Montano

Dynamic layouts with Spec2

As you may already know, Spec2 is the new version of the UI framework: Spec. Spec2 is not just a new version but a complete rewrite and redesign of Spec1. Contrary to Spec1, in Spec2 all the layouts are dynamic. It means that you can change on the fly the elements displayed. It is a radical improvement from Spec1 where most of the layout were static and building dynamic widgets was cumbersome.

In this post we will show that presenters can be dynamically composed using layouts. We will show a little interactive section. Then we will build a little code editor with dynamic aspects. Note that In this post, we are going to use simply Spec, to refer to Spec2 when we do not need to stress a difference.

Layouts as simple as objects

Building dynamic applications using Spec is simple. In fact, any layout in Spec is dynamic and composable. For example, let me show you the following code snippet:

"Instantiate a new presenter"
presenter := SpPresenter new.
"Optionally, define an application for the presenter"
presenter application: SpApplication new.

There are three principal layouts in Spec: SpPanedLayout, SpBoxLayout and SpGridLayout. For this presenter we will use the SpPanedLayout, which can receive two presenters (or layouts) and places them in one half of the window.

presenter layout: SpPanedLayout newTopToBottom.
presenter openWithSpec.

Of course, we are going to see an empty window because we did not put anything in the layout.

Empty layout

Now, without closing the window, we can dynamically edit the layout of the main presenter. We will add a button presenter executing the following lines:

presenter layout add: (button1 := presenter newButton).
button1 label: 'I am a button'.
Paned layout with one button

Now, we can add another button. There is no need to close and reopen the window, everything updates dynamically and without the need of rebuilding the window. As we instantiate the layout with newTopToBottom, the presenters will align vertically.

presenter layout add: (button2 := presenter newButton).
button2 label: 'I am another button'.
Paned layout with two buttons

Now, we can put an icon for the first button:

button1 icon: (button1 iconNamed: #smallDoIt).
Paned layout

Or we can delete one of the buttons from the layout:

presenter layout remove: button2.

What we should see here is that all the changes happens simply by creating a new instance of a given layout and sending messages to it. It means that programs can create simply complex logic of the dynamic behavior of a widget.

Building a little dynamic browser

Now, with all of this knowledge, we are going to build a new mini version of the System Browser. We want to have

  • A tree that shows all the system classes.
  • A list that shows all methods in the selected class.
  • A text presenter that show the code of a selected method and a button.
  • Initially the code of the method will be in “Read-Only” mode. When we press the button, we are going to pass to “Edit” mode.

Let us get started. So, first, we need to create a subclass of SpPresenter, called MyMiniBrowserPresenter.

SpPresenter subclass: #MyMiniBrowserPresenter
	instanceVariableNames: 'treeClasses button codeShower methodsList'
	classVariableNames: ''
	package: 'MiniBrowser'

Now, we need to override the initializePresenters method in which we are going to initialize the presenters and the layout of our mini browser.

First we are going to instantiate the tree presenter. We want the tree presenter to show all the classes that are presented in the Pharo image. We know that all subclasses (almost) inherit from Object. So, that is going to be the only root of the tree. To get the subclasses of a class we can send the message subclasses, that is what we need to get the children of a node. We want to each of the nodes (clases) have a nice icon, we can get the icon of a class with the message systemIcon. Finally, we want to “activate” the presenter with only one click instead of two. The code will be:

MyMiniBrowserPresenter >> initializePresenters

    treeClasses := self newTree.
	   roots: Object asOrderedCollection;
	   children: [ :each | each subclasses ];
	   displayIcon: [ :each | each systemIcon ].

For the methods, we want to have a filtering list. That means, a list in which we can search of elements. Also, we want that to display only the selector of the method to the user and sort them in an ascending way.

    methodsFilteringList := self newFilteringList.
    methodsFilteringList display: [ :method | method selector ].
    methodsFilteringList listPresenter
	    sortingBlock: [ :method | method selector ] ascending.

We said that, initially, the code is going to be in “Read-Only” mode. So, the label of the button is going to be “Edit” so say that is we click on the button we will change to edition mode. Also we want to have a nice icon.

   button := self newButton.
	  label: 'Edit';
	  icon: (self iconNamed: #smallConfiguration).

As the initial behaviour will be read-only mode, the code shower will be only a text presenter that is not editable.

   codeShower := self newText.
   codeShower beNotEditable.

And finally we want to intialize the layout of our presenter.

self initializeLayout

Here the complete code of the method is:

MyMiniBrowserPresenter >> initializePresenters

	treeClasses := self newTree.
		roots: Object asOrderedCollection;
		children: [ :each | each subclasses ];
		displayIcon: [ :each | each systemIcon ].

	methodsFilteringList := self newFilteringList.
	methodsFilteringList display: [ :method | method selector ].
	methodsFilteringList listPresenter
		sortingBlock: [ :method | method selector ] ascending.

	button := self newButton.
		label: 'Edit';
		icon: (self iconNamed: #smallConfiguration).

	codeShower := self newText.
	codeShower beNotEditable.

	self initializeLayout

Placing elements visually

We want in the upper part of the layout to have the classes and the methods shown in a horizontal way, like in the System Browser (a.k.a. Calypso). So, what we will do is to create another left to right layout, with an spacing of 10 pixels, the classes and the methods.

Then, we will add that layout to our main layout. the main layout is going to be a top to bottom layout. After, we want the code shower and then the button. We do not want the code to expand and also we want a separarion of 5 pixels for this layout.

MyMiniBrowserPresenter >> initializeLayout

	| classesAndMethodsLayout |
	classesAndMethodsLayout := SpBoxLayout newLeftToRight.
		spacing: 10;
		add: treeClasses;
		add: methodsFilteringList.
	self layout: (SpBoxLayout newTopToBottom
		spacing: 5;
		add: classesAndMethodsLayout;
		add: codeShower;
		add: button expand: false;

So far, so good… but we did not add any behaviour to the presenters. To do that we can either do it in the initializePresenters method of override the connectPresenters method. To clearly separate the intention of the methods, we favor overriding connectPresenters.

Connecting the flow

When we click on a class of the tree, we want to update the items of the methods list with the methods of the selected class. When we click on a method, we want to update the text of the code shower with the source code of the method.

MyMiniBrowserPresenter >> connectPresenters

	treeClasses whenActivatedDo: [ :selection | 
		methodsFilteringList items: selection selectedItem methods ].
	methodsFilteringList listPresenter
		whenSelectedDo: [ :selectedMethod | 
			codeShower text: selectedMethod ast formattedCode ].
	button action: [ self buttonAction ]

When we click on the button we want several things. That is why it is better to create a separated method. First, we want to change to label to the button to alternate between “Edit” and “Read-Only”. Then, we want to change the presenter of the code shower. If the Mini Browser is on read only mode we want to have a text presenter that is not editable. And if the Mini Browser is on edit mode we want to have a code presenter that highlights the code and show the number of lines of code. But always the code shower is going to have the same text (the code of the methods).

MyMiniBrowserPresenter >> buttonAction

	| newShower |
	button label = 'Edit'
		ifTrue: [ 
			button label: 'Read only'.
			newShower := self newCode ]
		ifFalse: [ 
			button label: 'Edit'.
			newShower := self newText beNotEditable ]

	newShower text: methodsFilteringList selectedItem ast formattedCode.

	self layout replace: codeShower with: newShower.
	codeShower := newShower

As a last detail, because we love details, we do not want the “Untitled window” as the window title and also we want a default extent. We override initializeWindow:method.

MyMiniBrowserPresenter >> initializeWindow: aWindowPresenter

		title: 'My Mini Browser';
		initialExtent: 750 @ 650

Voilà! We have a new version minimal version of the System Browser. If we run MyMiniBrowserPresenter new openWithSpec.

Mini Browser on Read-Only mode
Mini Browser on Edit mode

With Spec we can build from simple applications to very sophisticated ones. The dynamic properties are simply nice. Spec has lots of presenters that are ready to be used. Start digging into the code to see with presenters are available, what it is their API and start experimenting and playing! Layouts can be configured in multiple ways, so have a look at their classes and the example available.

Sebastian Jordan-Montano

Debugging the JIT compiler Hotspot detection for ARM64

The other day we were working on the compiler detection of hotspots, originally implemented by Clément Béra during his PhD thesis. In Sista, hotspot detection is implemented as a countdown that looks like the following: the counter is loaded in a register, decremented and then a jump if carry detects if the substraction underflowed.

self MoveA32: counterAddress R: counterReg.
self SubCq: 16r10000 R: counterReg. "Count executed"
countTripped := self JumpCarry: 0.

We were building some unit tests for this functionality, and we were interested at first at seeing how counters increment/decrement when methods execute. We wrote a couple dozen of tests for different cases (because the code for the counters is a bit more complicated, but that’s for another day). The code of one of our tests looked like the following: we compile a method, we execute it on a machine code simulator, then we verify that the counter was effectively incremented (because the public API is in terms of positive counts and not count-downs):

    | nativeMethod counterData jumpCounter |
    nativeMethod := self jitMethod: (self class>>#methodWithAndAndJump:).
        callCogMethod: nativeMethod
        receiver: memory nilObject
        arguments: { memory trueObject }
        returnAddress: callerAddress.

    counterData := interpreter picDataFor: nativeMethod.
    jumpCounter := memory fetchPointer: 1 ofObject: counterData.
        assert: (memory integerValueOf: (memory fetchPointer: 1 ofObject: jumpCounter))
        equals: 1

There was however something fishy about the ARM64 version. In addition of incrementing the counter, the code was taking the carry jump! Which lead our test to fail…

Doing some machine code debugging

So everything was working OK on intel (IA32, X64) but not on ARM (neither 32 or 64 bits). In both ARM versions the jump was __incorrectly__ taken. I first checked the instruction was being correctly assembled. And since that seemed ok, I went on digging in our machine code debugger. I found the corresponding instruction, set the instruction pointer in there and started playing with register values to see what was happening.

As you can see in the screenshot, the code is being compiled into subs x25, x25, x16, which you can read as x25 := x25 - x16. So I started playing with the values of those two registers and the carry flag, which is the flag that activates our jump carry. The first test I did was to check 2 - 1.

self carry: false.
self x25: 2.
self x16: 1.

Substraction was correct, leaving the correct result in x25, but the carry flag was set! That was odd. So I tested a second thing: 0 - 1.

self carry: false.
self x25: 0.
self x16: 1.

In this case, the carry flag was not set, but the negative was set. Which was even more odd. The case that should set carry was not setting it, and vice-versa. It seemed it was inverted! I did a final test just to confirm my assumption: 1-1 should set both the negative and carry flags if the carry flag was inverted.

self carry: false.
self x25: 0.
self x16: 1.

ARM Carry is indeed strange

I was puzzled for a moment, and then I got to look for a culprit: was our assembler that was doing something wrong? was it a bug in Unicorn, our machine code simulator? or was is something else?

After digging for some time I came to find something interesting in the ARM documentation:

For a subtraction, including the comparison instruction CMP and the negate instructions NEGS and NGCS, C is set to 0 if the subtraction produced a borrow (that is, an unsigned underflow), and to 1 otherwise.

And something similar in a stack overflow post:

ARM uses an inverted carry flag for borrow (i.e. subtraction). That’s why the carry is set whenever there is no borrow and clear whenever there is. This design decision makes building an ALU slightly simpler which is why some CPUs do it.

It seems that the carry flag in ARM is set if there is no borrow, so it is indeed inverted! But it is only inverted for substractions!

Extending the Compiler with this

Since carry works different in different architectures, but only for substractions, I created a new instruction factory method JumpSubstractionCarry: that detects carry for substractions and is supposed to be platform specific. Then I replaced the code of the counter by the following:

self MoveA32: counterAddress R: counterReg.
self SubCq: 16r10000 R: counterReg.
countTripped := self JumpSubstractionCarry: 0.

JumpSubstractionCarry: is backend defined:

JumpSubstractionCarry: jumpTarget
    backEnd genJumpSubstractionCarry: jumpTarget

the default implementation just delegates to the original factory method:

genJumpSubstractionCarry: jumpTarget
    ^cogit JumpCarry: jumpTarget

and the ARM (both 64 and 32 bits) do use a jump if no carry instead!

genJumpSubstractionCarry: jumpTarget
    ^cogit JumpNoCarry: jumpTarget

With these, our tests went green for all platforms 🙂

If you want to see the entire related code, you can check the following WIP:…guillep:sista?expand=1

Advanced stepping and custom debugging actions in the new Pharo debugger

In this article, we describe the new advanced stepping menu available in the debugger. These advanced steps provide convenient and automated debugging actions to help you debug your programs. We will see how to build and add new advanced steps, and how to extend the debugger toolbar with your own customized debugging menus.

Advanced steps

Have you noticed the bomb in the debugger toolbar? These are the advanced steps! These steps provide you with usefull and convenient debugging actions. Basically, they automatically step the execution until a given condition is satisfied. When it is the case, the execution is interrupted and its current state is shown in the debugger. In the following, we will describe these advanced steps and implement and integrate a new advanced step that skips the current expression.

Advanced steps menu in the debugger tool bar

What do these advanced steps do?

These advanced steps are a bit experimental (notice the bomb!). This means they can sometimes be a bit buggy, and in that case you should open an issue to report the bug. However most of the time, they do the job. Some of them have a failsafe that stops to give feedback to developers, in order to avoid an infinite stepping due to the impossibility to meet the expected conditions. For now, that failsafe limits the automatic stepping to 1000 steps before notifying the developer and asking if she wants to continue. The current advanced steps and what they do are describe below.

Steps until any instance of the same class of the current receiver receives a message. For instance, the receiver is a point executing the extent message. This command will step until another point receives a message.

Steps until the current receiver receives a message. For example, the next time a visitor is called back from a visited object.

Steps until the execution enters a new method. Stops just after entering that new method.

Steps the execution until the next object creation, that is, the next class instantiation.

Steps the execution until the current method is about to return. Stops just before returning.

Building a new advanced step: skipping expressions

In the following, we build a new advanced step to demonstrate how you can easily add new debugging commands to the advanced step menu.

Building the command class

First, we must create your class as a subclass of SindarinCommand. The SindarinCommand class provides small facilities to build debugger commands, such as accessors to the debugger API and to the debugger UI.

SindarinCommand subclass: #SindarinSkipCommand
    instanceVariableNames: ''
    classVariableNames: ''
    package: 'NewTools-Sindarin-Commands'

Second, we must write three class methods to configure the command: you have to provide an icon name, a description and a name. The defaultName method also contains the pragma <codeExtensionDebugCommand: 50>: this pragma is how the debugger automatically finds the command to display it in the advanced steps menu. The parameter of the pragma is the order of appearance of the menu action (we will not bother with it in this tutorial).

SindarinSkipCommand class>>defaultIconName
SindarinSkipCommand class>>defaultDescription
	^ 'Skips the current expression'
SindarinSkipCommand class>>defaultName
	<codeExtensionDebugCommand: 50>
	^ 'Skip'

If we open a new debugger, we see now that a new advanced step is available: the skip debugging action.

The new “Skip” advanced step automatically appeared in the menu.

Building the skip action

Now that we have our menu button, we need to write what it does! We must write the execute method in the SindarinSkipCommand. This method is the one called every time you click on an advanced step button.

Ideally, commands should not contains the logic of the debugging code because it requires to access and modify elements from the debugger UI (or debugger presenter) and to access and control the debugging model. This is not always possible (everything is not accessible from outside the debugger) and this also leads to complex and hard to test code in those execute methods.

That is why we provide an access to the debugger UI through the debuggerPresenter accessor, and that we only call its API in those execute commands. In our implementation below, we call the skipCurrentExpression API that implements the skipping behavior. We do not show this implementation here as our focus is the adding of new advanced steps. In addition, we prefer to create the skipCurrentExpression API as an extension method of the debugger presenter and located in the same package as our skip command class.

	self debuggerPresenter skipCurrentExpression

Experimenting our new skip action

We see a demonstration of this new advanced step in the video below. Notice that everything is not possible: at the end, the debugger refuses to skip the return of the method.

Additionally, skipping code is a sensible operation. It can lead to an inconsistent program state, and you must use it with caution. Remember: there is a bomb in the menu 🙂

How to build your own debugger action by extending the debugger action bar

Extending the toolbar of the debugger with your own menu and commands is fairly easy. You can do it in a few steps, that we describe below.

First, we need to create an extension method of the debugger that will be automatically called by the Spec command building mechanics. This methods takes two parameters: stDebuggerInstance as the debugger instance requesting to build commands, and rootCommandGroup, the default command tree built by that debugger instance. The first instruction of this extension method is the <extensionCommands> pragma. Spec uses this pragma to find all methods extending the command tree of a given presenter (here the debugger) to automatically build extensions.

This method starts like this:
StDebugger>>buildMyExtentionMenuWith: stDebuggerInstance forRoot: rootCommandGroup

Now, let us assume that you built a set of commands, that we refer to as yourCommandClasses in the following. We instantiate all your commands and store them into a commands temporary variable. Each time, we pass the debugger instance to the instantiated command. All these commands can then obtain a reference to the debugger by executing self context, which returns the debugger, and use its API.

commands := yourCommandClasses collect: [:class | class forSpecContext: stDebuggerInstance ].

The next step is to obtain the toolbar command tree from the debugger command tree. This tree contains all the default commands of the debugger, that we want to extend:
toolbarGroup := rootCommandGroup / StDebuggerToolbarCommandTreeBuilder groupName.

Then, we build our own command group and we add this group to the toolbar. The following code configures that new group as a menu button that opens with a popover (as for advanced steps described above):
yourToolbarGroup := CmCommandGroup forSpec
name: 'Advanced Step';
icon: (stDebuggerInstance application iconNamed: #smallExpert);

toolbarGroup register: yourToolbarGroup.

Finally, we register our commands to our new command group, which will make then available in the debugger toolbar:
commands do: [ :c | yourToolbarGroup register: c ].

The full method looks like this:

StDebugger>>buildMyExtentionMenuWith: stDebuggerInstance forRoot: rootCommandGroup 
commands := yourCommandClasses collect: [:class | class forSpecContext: stDebuggerInstance].
yourToolbarGroup := CmCommandGroup forSpec
    name: 'Advanced Step';
    icon: (stDebuggerInstance application iconNamed:     #smallExpert);
toolbarGroup register: yourToolbarGroup.
commands do: [ :c | toolbarSindarinGroup register: c ].


We have seen the advanced steps, what they do, and how we can build and add new advance steps. We have then see how to extend the debugger toolbar with our own customized debugger actions.

Now, you have more power over your debugger, and you can use it to build awesome debugging tools suited to your own problems!

Installing Pharo in Linux using the System Package Manager

One of the improvements that we are including in Pharo 9 is the update of the build process in OpenBuildService.

This service allows us to produce packages for different distributions of Linux. These pacakges are built using the versions loaded in the distribution and they can be installed and updated using the tools present in the system.

Currently we have support for the following set of distributions and architectures, but more are coming.

  • Debian: 9.0 / 10.0 (X86_64)
  • Ubuntu: 18.04 / 20.04 / 20.10 (X86_64, ARM v8 (64 bits))
  • Raspbian: 9.0 / 10.0 (X86_64, ARM v7(32 bits), (X86_64, ARM v7))
  • Arch (X86_64)

We still have the version as latest, later they will be pass to stable as soon as we release Pharo 9.

Updated instructions for installing are found here:

A Taste of Ephemerons

For a couple of years now, Pharo includes support for Ephemerons, originally introduced with the Spur memory manager written by Eliot Miranda. For the upcoming Pharo 9.0 release, we have stressed the implementation (with several hundred thousands Ephemerons), make it compatible with latest changes done in the old space compaction algorithm, and made it a tiny bit more robust. In other words, from Pharo 9 and on, Ephemerons will be part of the Pharo family for real, and we will work on Pharo 10 to have a nice standard library support for it. For now, the improvements are available only in the latest night build of the VM, waiting to be promoted as stable.

Still, you would be scratching your head at “what the **** are ephemerons?”. The rest of this post will give a taste of them.

What are Ephemerons?

An ephemeron is a data structure that gives some notification when an object is garbage collected, invented by Barry Hayes and published in 1997 in OOPSLA in a paper named “Ephemerons: A New Finalization Mechanism”. This mechanism is particularly useful when working, for example, with external resources such as files or sockets.

To be concrete, imagine you open a file, which yields an object having a reference to a system’s file descriptor. You read and write from it, and when you’re done, you close it. Closing the file closes the file descriptor and returns the ressource to the OS. You really want your file to be closed, otherwise nasty stuff may happen, because your OS will limit the number of files you can open.

Sometimes however, applications do not always have such a straight and simple control flow. Let’s imagine the following, not necessarily realistic, arguably not well designed, but very illustrative case: Sometimes you open a file, you pass your file as argument to some library, and… now the library owns a reference to your file. So maybe you don’t want to close it yet. And the library may not want to close it either because you are the real owner of the file!

Another possibility is to let the file be. And make sure that when the object is not used anymore and garbage collected, we close its file descriptor. An Ephemeron does exactly that! It allows us to know when an object is collected, and gives us the possibility to “finalize” it.

You can test it doing (using the latest VM!):

Object subclass: #MyAnnouncingFinalizer
    instanceVariableNames: ''
    classVariableNames: ''
    package: 'MyEphemeronTest'

MyAnnouncingFinalizer >> finalize [
    self inform: 'gone!'

obj := MyAnnouncingFinalizer new.

e := Ephemeron new.
e key: obj.

obj := nil

You will see that after nilling the variable obj, the Ephemeron will react and send the finalize message to our MyAnnouncingFinalizer object.

What about weak objects?

Historically Pharo also supports weak objects, and another finalization mechanism for them.
A weak object is a special kind of object whose references are weak.
And to say it informally, a weak reference is an object reference that is not taken seriously by the garbage collector. If the garbage collector finds that an object is only referenced by weak references, it will collect it, and replace all those weak references by a “tombstone” (which tends to be nil in many implementations).

Historically, we have used the weak mechanism for finalization in Pharo, which can be used like this:

obj := MyAnnouncingFinalizer new.
weakArray := WeakArray new: 1.
weakArray at: 1 put: obj.
WeakRegistry default add: obj.

Here, the weak array object will have a weak reference to our object, and the obj reference in the playground will be a strong reference. As soon as we nil the playground reference, the object will be detected for finalization and it will execute the finalize method too. Moreover, if we check our weak array, we will see our tombstone there in place of the original object.

obj := nil.

weakArray at: 1.

Why not using this weak finalization instead of the ephemeron one?
The main explanation is performance. With the weak finalization process, every time the VM detects an object needs to be finalized, it raises an event. Then, the weak finalization library will iterate all elements in the registry checking what elements need to be finalized, by looking for the presence of tombstones. This means that for each weak object the weak finalization must do a full scan of all possible registered weaklings!

The ephemeron mechanism is more direct: when the VM detects an ephemeron needs to be finalized, it will push the ephemeron to a queue, and raise an event. Then, the ephemeron finalization will empty the queue and finalize them. No need to check all existing ephemerons.

A Weak Pharo Story, Memory Leaks and More

Of course, ephemerons are not only necessary for efficiency. They help also avoid many nasty memory leaks. A couple of years ago we did with Pavel a presentation in ESUG about a very concrete memory leak caused by mis-usage of weak objects. It’s a fun story to tell with enough perspective, but it was not a fun bug to track down at the time 😛 .

And even more, a robust ephemeron implementation will help us remove all the (potential buggy and inefficient) weak finalization code in Pharo 10!

Debugger Extensions in Pharo 9.0

Did you ever want to get the value of an expression each time you navigate the debugger stack? This is what we will show in this tutorial: you’ll learn how to implement a new extension for the StDebugger in Pharo9.0: the ‘Expression Evaluator StDebugger Extension’ (or simply EvaluatorDebugger for short) – that allows the evaluation and inspection of an arbitrary expression in any of the available contexts.

Navigation links

I. Introduction: Outline of the debugging events and debugger extensions.
II. Tutorial Part I: Creating a basic empty debugger extension.
III. Tutorial Part II: Implementing an Expression Evaluator Debugger Extension.
The finished code can be found in its repository

I. Introduction

Debugging in Pharo 9.0

Whenever you debug something in Pharo, this is what happens.

  1. Pharo choses an appropriate debugger.
    It’ll be the StDebugger in most scenarios.
  2. Once a debugger is chosen by the runtime, several things happen:
    • The UI object for the chosen debugger is instantiated (StDebugger).
    • The Debug Process is started and a DebugSession object is instantiated.
    • An action model object (StDebuggerActionModel) is instantiated.
    • The associated extensions for the debugger are loaded.

The general idea is that the debugger (UI) interacts with the action model. The action model is an interface to the debug session, which owns and works over a debug process.

To understand a little bit better, here is a little explanation of each one of the actors (objects) relevant for creating a debugger extension.

I.1 StDebugger (The debugger UI)

The StDebugger class inherits from SpPresenter and acts as the main UI for the debugger. It owns the following objects:

  • DebugSession.
  • StDebuggerActionModel

The UI object it’s usually designed to allow the usage of the functionalities exposed by the ActionModel object.

I.2 DebugSession

The DebugSession models a debugging session by holding its state and providing a basic API to perform elemental debugging operations. It allows one, among others actions, to:

  • Step the execution.
  • Manipulation of Contexts.

It owns the Debug Process.

I.3 Debug Process

It’s the process that the DebugSession will work upon. It runs the debugged execution. It’s owned by the DebugSession object.

I.4 ActionModel

Your debugger extension logic should not be implemented directly in the presenter (the UI). To separate responsibilities, we code an ActionModel object, which will implement the complex execution behavior based on the Debug Session.

StDebuggerActionModel exposes all the functions that are available for the StDebugger.

I.5 StDebugger Extensions

When the StDebugger is initialized, it loads all its extensions. A debugger extension is minimally composed by the following:

  1. A presenter object (UI).
    The UI, a subclass of SpPresenter, that allows the user the make use of the extension capabilities. Note that having just a presenter is not enough. For this object to be recognized and listed as a Debugger Extension, the class must use the Trait: TStDebuggerExtension.
  2. An ActionModel object.
    This is the object that implements and exposes all its special debugging functionalities. It’s normally owned by the extension presenter.

In this tutorial, you will create a Presenter and an ActionModel for your debugger extension.

II. Tutorial – Part I

In “Tutorial Part I”, you will develop a minimal (blank, no tests, no accessors, no debugging features yet) implementation of a debugger extension, that is integrated with the Debugger Extensions system in Pharo 9.

An blank Debugger Extension is composed by an UI and an ActionModel object .

Adding a new debugger extension for the StDebugger

The first step towards a fully featured new extension is to create an empty one that is integrated with the debugger extensions system of Pharo 9, and it’s shown among the other extensions.

You will learn how to implement the following:

  • The extension UI (SpPresenter Subclass, with TStDebuggerExtension trait).
  • The Extension Action Model (Your actual Debugger Extension object that exposes its functionalities).
The finished debugger extension

Implementing the basics for debugger extensions

The Action Model

In your package of choice (Here called ‘EvaluatorDebugger-Base’), define a class for your debugger extension model. Name it “EvaluatorDebugger”. There are no constraints related to the design, but it is a good idea to hold a reference, to the StDebugger, the DebugSession, or whatever your extension might need.

Note: You are not developing a Debugger, but a Debugger Extension. Nonetheless, we call it EvaluatorDebugger for simplicity reasons.

Object subclass: #EvaluatorDebugger
   instanceVariableNames: 'stDebugger'
   classVariableNames: ''
   package: 'EvaluatorDebugger-Base'

Add also the accessors.

EvaluatorDebugger >> stDebugger
   ^ stDebugger

EvaluatorDebugger >> stDebugger: anObject
   stDebugger := anObject
The Extension UI

In your package of choice, create a subclass of SpPresenter that uses the trait TStDebuggerExtension. Name it “EvaluatorDebuggerPresenter”. Also, add an instance variable to hold your Action Model:

SpPresenter subclass: #EvaluatorDebuggerPresenter
   uses: TStDebuggerExtension
   instanceVariableNames: 'evaluatorDebugger'
   classVariableNames: ''
   package: 'EvaluatorDebugger-Base'

Remember to implement the trait methods; in particular, put a name to your extension.

EvaluatorDebuggerPresenter >> debuggerExtensionToolName
   ^ 'Evaluator Debugger' 

The Extensions system relies on certain methods to be implemented in your UI object to have a functional extension. Implement the following:

EvaluatorDebuggerPresenter >> setModelBeforeInitialization: aStDebugger
   "This method is called when the StDebugger initializes its extensions.
   We initialize our model (the debugger extension) with a reference to the stDebugger."
   evaluatorDebugger := EvaluatorDebugger new.
   evaluatorDebugger stDebugger: aStDebugger

EvaluatorDebuggerPresenter >> initializePresenters
   "Called automatically by the Spec framework. This method describes how the widgets are initialized"
   "There are no widget for the moment."

EvaluatorDebuggerPresenter >> updatePresenter
   "Called automatically when the debugger updates its state after stepping"
   "Your widgets should be updated here."
   super updatePresenter

And in the class side, your presenter needs the following method to be implemented:

EvaluatorDebuggerPresenter class >> defaultSpec
"An empty vertical box layout, for the moment"
    ^ SpBoxLayout newVertical

So far, you have an empty debugger extension. It doesn’t do anything yet.

Next, you’ll make it appear among the other extensions.

Activate the debugger extensions in Pharo

By default, there are no debugger extension being shown.
How to see your new extension?

So far, you have a functional empty debugger extension. For it to be visible and available in the StDebugger, you need to enable the Debugger Extensions in the Pharo Settings. This is how:

  1. Go to the Pharo Settings.
  2. Navigate to Tools > Debugging > Debugger Extensions and check the option Activate Extensions…
  3. Expand Activate Extensions… and find your extension (Evaluator Debugger) check the option Show in Debugger.

Additionally When developing debugger extensions, it is recommended to enable the option to Handle Debugger Errors, like in the last picture.

By default, if your debugger extension throws an error, it will be ignored and the StDebugger will not load the extension. This means that you can’t debug your extension code directly in case of failure. By enabling Handle Debugger Errors, whenever an error is thrown in your extension, a new StDebugger(without extensions) instance will be launched so you can debug it.

For this, navigate and check the option: Tools > Debugging > Handle Debugger Errors.

From now on, whenever you debug something, your extension should appear in the top-right pane.

Your empty debugger extension, shown in the StDebugger

III. Tutorial – Part II

Implementing the Expression Evaluator StDebugger Extension

Remember: For readability purposes, in this tutorial the extension is called simply by “EvaluatorDebugger” instead of its full connotation: “Expression Evaluator StDebugger Extension”

Now you’ll add your extension functionalities. For this, you will:

  1. Implement the logic of your debugger extension (Implement the ActionModel – EvaluatorDebugger – methods).
  2. Implement an object subclass of SpCodeScriptingInteractionModel – EvaluatorDebuggerCodeInteractionModel – needed for the expression-evaluation-in-context logic.
  3. Finish the UI (EvaluatorDebuggerPresenter) layout and widgets.

During the Tutorial – Part I, we developed an Action Model without any behavior – The EvaluatorDebugger. This time, we will complete the class with the intended logic by adding a method that allows the evaluation of an expression in a given Context.

Add a new method: #evaluateInCurrentContextExpression:withRequestor:

EvaluatorDebugger >> evaluateInCurrentContextExpression: aStream withRequestor: anObject
   "Evaluates the expression coming from a stream. Uses the current context of the StDebugger"
   | context |
   context := stDebugger currentContext.
   ^ context receiver class compiler
        source: aStream;
        context: context;
        receiver: context receiver;
        requestor: anObject;
        failBlock: [ nil ];

Your extension UI will feature a SpCodePresenter, where the user can type an expression which is evaluated in the selected context of the StDebugger.

Your code presenter should consider the current selected context to correctly work with your code (Syntax highlighting, inspection, etc), and for this you need to implement a subclass of SpCodeScriptingInteractionModel as follows.

SpCodeScriptingInteractionModel subclass: #EvaluatorDebuggerCodeInteractionModel
   instanceVariableNames: 'context'
   classVariableNames: ''
   package: 'EvaluatorDebugger-Base'
EvaluatorDebuggerCodeInteractionModel >> bindingOf: aString
   ^ (context lookupVar: aString) ifNotNil: [ :var | 
        var asDoItVariableFrom: context ]

EvaluatorDebuggerCodeInteractionModel >> context
   ^ context

EvaluatorDebuggerCodeInteractionModel >> context: anObject
   context := anObject

EvaluatorDebuggerCodeInteractionModel >> hasBindingOf: aString
   ^ (context lookupVar: aString) notNil

Finally, define the layout and behavior of the extension UI.

UI Layout

The object is a Spec-based presenter. Design a neat and practical interface!

Modify the EvaluatorDebuggerPresenter class. Add the instance variables for the widgets.

SpPresenter subclass: #EvaluatorDebuggerPresenter
   uses: TStDebuggerExtension
   instanceVariableNames: 'toolbar code inspector valueLabel evaluatorDebugger'
   classVariableNames: ''
   package: 'EvaluatorDebugger-Base'

Define the layout in the class side as follows.

EvaluatorDebuggerPresenter class >> defaultSpec
^ SpBoxLayout newVertical
      add: #toolbar expand: false fill: false padding: 0;
      add: #code;
      add: 'Expression Value' expand: false fill: false padding: 5;
      add: #valueLabel expand: false fill: false padding: 5;
      add: #inspector;

Implement the following instance-side methods.

EvaluatorDebuggerPresenter >> initializeCode
   "We define the extensions Code presenter initialization here"
   code := self newCode.
   code interactionModel: EvaluatorDebuggerCodeInteractionModel new.
   code syntaxHighlight: true.
   code text: '"put your expression here"'

EvaluatorDebuggerPresenter >> initializePresenters
   "Called by the Spec framework. This method describes how the widgets are initialized"
   self initializeToolbar.
   self initializeCode.
   valueLabel := self newLabel.
   valueLabel label: 'Write an expression first'.
   inspector := nil inspectionRaw.
   inspector owner: self.

   "when changing the selected context in the stDebugger stackTable, re-evaluate the expression in that context"
   evaluatorDebugger stDebugger stackTable selection whenChangedDo: [ 
      self updatePresenter ].
   self updatePresenter

EvaluatorDebuggerPresenter >> initializeToolbar
   toolbar := self newToolbar
                 addItem: (self newToolbarButton
                        icon: (self application iconNamed: #smallDoIt);
                        action: [ self updatePresenter ];


‘updatePresenter’ is meant to be automatically called when the debugger updates its state after stepping, and changing the context in the stack. However, the current version of Pharo9.0 – at the date of writing 2021/02/16 – doesn’t perform the update after changing the selected context. To fix this, we used the following “hacky” code:

evaluatorDebugger stDebugger stackTable selection whenChangedDo: [
self updatePresenter ]

and add an updatePresenter call to the table selection change callbacks collection in the method initializePresenters above.


The user will write expressions and “press buttons/click things” in the debugger and your extension, expecting something to happen. Also, the StDebugger might issue an “updatePresenter” call to all the extensions. You need to code that.

Add an accessor to directly expose the current context.

EvaluatorDebuggerPresenter >> currentStDebuggerContext
   "A 'shortcut' to get the same currentContext of the StDebugger"
   ^ evaluatorDebugger stDebugger currentContext

Remember that whenever the StDebbuger updates its state, it will automatically call updatePresenter for each of the extensions. We want the code presenter to reflect that, and also the displayed expression value.

EvaluatorDebuggerPresenter >> updatePresenter
   "Called automatically when the debugger updates its state after stepping"
   self updateCode.
   self updateExpressionValueDisplayed.
   super updatePresenter
EvaluatorDebuggerPresenter >> updateCode
   "Sets the context of our debugger-extension code presenter to be the same one of the StDebugger"
   code interactionModel context: self currentStDebuggerContext

EvaluatorDebuggerPresenter >> updateExpressionValueDisplayed
   "Evaluate the expression, in the code presenter, using the appropriate context (the current one of the stDebgger). Then update the ui to show and inspect the obtained value, or a potential exception."
   | expressionBlock expressionResult errorFlag errorMessage |
   expressionBlock := [ 
                         code text readStream
                         withRequestor: code interactionModel ].
   errorFlag := false.
   expressionResult := expressionBlock
                          on: Exception
                          do: [ :e | 
                             errorFlag := true.
                             errorMessage := e description.
                             e ].
   "The inspector shows the result object in case of success, or the Exception otherwise"
   inspector model: expressionResult.
   valueLabel label: (errorFlag
          ifTrue: [ errorMessage ]
          ifFalse: [ expressionResult asString ])

Try it!

Now you have a fully functional debugger-extension. Try debugging some code!

Example code to be debugged:


myCollection:= OrderedCollection new.
myCollection add: 1.
myCollection add: 2.
myCollection add: 3.
myCollection add: 4.

Object assert: myCollection size == 3
  1. Debug some code (cmd+D in the Playground).
  2. Write an expression in your extension’s code presenter (try the code in the image below, if following the example).
  3. Select different Contexts in the stDebugger and see what happens!


The Pharo 9.0 debugger extensions system allows one to conveniently add new ones. The example developed in the tutorial explores all the basic aspects needed to have a functional extension completely integrated with the environment. Should you need to create a new one, or modify and existing one, now you have the knowledge.