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Pharo Notes

Notes on the modern Smalltalk variant Pharo

  • One-line code examples can be executed using shortcuts without selecting them. Multi-line examples must be selected first.

  • == tests only for object equality (like in Java). More likely the user wants =.

  • "Literal arrays are created at parse time and are read-only" (but copies are mutable—presumably the isReadOnlyObject primitive code handles this detail).

  • Use dynamic arrays for instances like this one: { (2 + 3) . (6 * 6) }.

  • Dynamic arrays are mutable from the get-go and do not need to be sent copy.

  • Pharo works with the system clipboard. How's that for Smalltalk playing along nice with the host system? :)

  • Use & for logical "and" and | for logical "or". Logical "not" is a unary message of the same name. (Actually, I was told and: and or: are more idiomatic because they're shortcut operators.)

  • Indeed, Kent Beck's Guide to Better Smalltalk says that an expression like anArray size >= 10 and: [(anArray at: 10) isNil] is useful to guard from errors.

  • Sending the binary message @ to a number type along with another number type to create a Point object.

  • PolyMath implements complex number arithmetic!

  • Date today + 3 weeks and Date yesterday < Date today both work, very nice.

  • 4 between: 0 and: 10 and 1 max: 3 are good examples of keyword messages.

  • Keyword messages can be written across multiple lines. (And it looks like this applies to other messages as well.)

  • ProfStef perform: #next is the same as ProfStef next.

  • Remember "unary > binary > keyword", "left to right". There is no algebraic order of operations. All exceptions to the rules need to be implemented with parentheses.

  • These are the same:

    Transcript show: 'hello'.
Transcript show: 'Pharo'.
Transcript cr.

    and (uses the cascade operator):

    Transcript 
    show: 'hello';
    show: 'Pharo' ;
    cr.

  • Examples of block invocation:

    [:x | x + 2] value: 5.

[GTPlayground open] value.

[:x | x + 2] value: 10.

[:x :y| x + y] value: 3 value: 5.

  • Pharo blocks have lexical closure.

  • Example of an "if" statement:

    1 < 2
  ifTrue: [100]
  ifFalse: [42].

    It's just sending ifTrue:ifFalse to a Boolean type!

  • Examples of loops:

    1 to: 100 by: 3 do: [:i | Transcript show: i asString; cr].

100 to: 0 by: -2 do: [:i | Transcript show: i asString; cr].

1 to: 1 do: [:i | ProfStef next].

  • More nice loops:

    #(11 38 3 -2 10) do: [:each |
 Transcript show: each printString; cr].

"Some other really nice iterators"

#(11 38 3 -2 10) collect: [:each | each abs].

#(11 38 3 -2 10) collect: [:each | each odd].

#(11 38 3 -2 10) select: [:each | each odd].

#(11 38 3 -2 10) select: [:each | each > 10].

#(11 38 3 -2 10) reject: [:each | each > 10].

#(11 38 3 -2 10) 
     do: [:each | Transcript show: each printString]
     separatedBy: [Transcript show: '.'].

  • Double quotes denote comments in Pharo.

  • The implementation of the unary selectors method does something like methods in Ruby.

  • respondsTo: with a symbol is another useful bit of introspection.

  • Shift+Alt+Left Click brings up the halo on a Morph to get rid of a pesky morph.

  • There is a Vim mode for Pharo!

  • From the Discord:

    readyready15728: "I want to file a bug report for Pharo 9.0: whenever I delete text over multiple lines using the Backspace key in a Playground, the Transcript gets flooded with SubscriptOutOfBounds errors. I'm not sure I would be able to fix it on my own but I don't know exactly how to track down the error without a tool like grep. How can I find the offending call to Transcript show:?"

    Todd Blanchard: "SubscriptOutOfBounds is a class. Specifically, it is a subclass of Error. So I would probably get it up in the browser (type its name in a playground, select it, cmd-b for browse) and then hit the old 'class refs' button to see where it is used. That should give you some clues."

  • WelcomeHelp open opens the initial welcome help window on a fresh image.

  • Nice examples of reflection:

"You can inspect and change the system at runtime.

Take a look at method #ifFalse:ifTrue: source code of class True:"

(True>>#ifFalse:ifTrue:) sourceCode.

"Or just its comment:"

(True>>#ifFalse:ifTrue:) comment.

"Here's all the methods I implement:"

ProfStef selectors.

"Now let's create a new method to go to the next lesson:"

ProfStef class compile:'goToNextLesson
  self next'.

"Wow! I can't wait to use my new method!"

ProfStef goToNextLesson.
  • How to get rid of the Exercism logo:

I want an untainted background image of a Mechanicus Forge World and both the Pharo and Exercism logos have to go. The former can easily be removed using the system menu but the latter requires a little work. I found out where the logo is set by using Finder, setting the type of search to "Classes" and searching for "Exercism", at BaselineOfExercism>>setExercismLogo:

setExercismLogo
	| exercismLogo |
	exercismLogo := (AlphaImageMorph
		withForm:
		(ZnEasy
		getPng:
			'https://user-images.githubusercontent.com/38899847/48952150-634e9b80-eefe-11e8-890e-7a233d56df65.png'))
		layout: #scaledAspect;
		extent: 100 @ 100;
		position: 10 @ 135;
		lock.
	World addMorph: exercismLogo

I then figured out I could go into a Playground and inspect World submorphs in search of my quarry. There are two AlphaImageMorph objects in there with the Exercism logo and I sent them both the unary message delete for an unblemished background.

  • The Iceberg tutorial omitted the part where, after creating src, you select the newly created directory as the location for uploaded source.
  • From a thread about Iceberg:
Indeed. Here's how to get the repository for class MyClass:

repo := IceRepository registeredRepositoryIncludingPackage: MyClass package.

And then the path ist just

repo location.

display
  self displayOn: Display

Which is implemented as:

displayOn: aDisplayMedium
  self displayOn: aDisplayMedium at: 0@0
  • "Use an Array if you know the size of the collection when you create it and if the indices into the elements (the first argument to at: and at:put:) are consecutive integers between one and the size of the array."
  • OrderedCollection is more flexible.
  • Instances of OrderedCollection can be used as stacks and queues, not unlike Python lists.
  • The Interval class also exists, similar to Python ranges.