The document discusses lexical scoping and returning values in blocks in Smalltalk. It explains that variables in blocks are bound in the context where the block is defined, not where it is evaluated. Continuation blocks can return values to the method that activated the home context. Returning from blocks requires care, as returning from within a block actually returns from the enclosing method. Examples demonstrate block evaluation, scoping, and using continuation blocks to return values or escape block evaluation.

1. S.Ducasse 1
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Stéphane Ducasse
Stephane.Ducasse@univ-savoie.fr
http://www.listic.univ-savoie.fr/~ducasse/
Fun with Blocks in VW

2. S.Ducasse 2
License: CC-Attribution-ShareAlike 2.0
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3. S.Ducasse 3
• VM represents the state of execution as Context
objects
• for method MethodContext
• for block BlockContext
• aContext contains a reference to the context from
which it is invoked, the receiver arguments,
temporaries in the Context
• We call home context the context in which a block is
defined
Advanced Blocks inVW

4. S.Ducasse 4
• Arguments, temporaries, instance variables are
lexically scoped in Smalltalk
• These variables are bound in the context in which the
block is defined and not in the context in which the
block is evaluated
Lexical Scope

5. S.Ducasse 5
Test>>testScope
"self new testScope"
|t|
t := 15.
self testBlock: [Transcript show: t printString]
Test>>testBlock:aBlock
|t|
t := 50.
aBlock value
Test new testBlock
-> 15 and not 50
Lexical Scope by Example

6. S.Ducasse 6
• ^ should be the last statement of a block body
[ Transcript show: 'two'.
^ self.
Transcript show: 'not printed']
• ^ return exits the method containing it.
• test
• Transcript show: 'one'.
• 1 isZero
• ifFalse: [ 0 isZero ifTrue: [ Transcript show: 'two'.
^ self]].
• Transcript show: ' not printed'
• -> one two
Returning from a Block

7. S.Ducasse 7
• Simple block [:x :y| x*x. x+y] returns the value of the
last statement to the method that sends it the
message value
• Continuation blocks [:x :y| ^ x + y] returns the value
to the method that activated its homeContext
• As a block is always evaluated in its homeContext, it is
possible to attempt to return from a method which
has already returned using other return.This runtime
error condition is trapped by theVM.
• Object>>returnBlock
• ^[^self]
• Object new returnBlock
• -> Exception
Returning From a Block (II)

8. S.Ducasse 8
|b|
b:= [:x| Transcript show: x. x].
b value:‘ a’. b value:‘ b’.
b:= [:x| Transcript show: x. ^x].
b value:‘ a’. b value:‘ b’.
Continuation blocks cannot be executed several times!
Continuation Blocks

9. S.Ducasse 9
Test>>testScope
|t|
t := 15.
self testBlock: [Transcript show:‘*’,t printString,‘*’.
^35 ].
^ 15
Test>>testBlock:aBlock
|t|
t := 50.
aBlock value.
self halt.
Test new testBlock
print: *15* and not halt.
return: 35
Example of Block Evaluation

10. S.Ducasse 10
|val|
val := [:exit |
|goSoon|
goSoon := Dialog confirm: 'Exit now?'.
goSoon ifTrue: [exit value: 'Bye'].
Transcript show: 'Not exiting'.
'last value'] myValueWithExit.
Transcript show: val.
val
yes -> print ‘Bye’ and return ‘Bye’
no -> print ‘Not Exiting’2 and return ‘2’
BlockClosure>>myValueWithExit
self value: [:arg| ^arg ].
^ ‘2’
BlockClosure>>myValueWithExit
^ self value: [:arg | ^ arg] to get no -> print ‘NotExiting’lastValue
Creating and escape mechanism

11. S.Ducasse 11
Summary
Scary and costly but fun!