Blocks - the Friends of Conditionals and Loops
Damien Cassou, Stéphane Ducasse and Luc Fabresse
http://stephane.ducasse.free.fr
Remember: A Block Definition Freezes its Body
- Evaluating a block definition does not execute its body
[ 2 + 6 ]
> [ 2 + 6 ]
Remember: Block Execution
- Executing a Block is explicit
[ 2 + 6 ] value
> 8
| b |
b := [ 2 + 6 ].
b value.
> 8
b value
> 8
Blocks are Used to Express Conditions
max: anObject
"Answer the receiver or the argument, whichever has the greater anObject."
^ self > anObject
ifTrue: [ self ]
ifFalse: [ anObject ]
Yes this is a message ifTrue:ifFalse: sent to a Boolean
Blocks are Used to Express Loops
- Some simple loops
- Printing 10 dots
10 timesRepeat: [ File stdout << '.' ]
> ............
Blocks are Used to Express Loops (2)
1 to: 10 do: [:i | File stdout << i ]
> 12345678910
Blocks are Used to Express Loops (3)
- A traditional
for loop for i=1,100, i++
1 to: 100 by: 3 do: [:i | File stdout << i ]
> 147101316192225283134374043464952555861646
> 770737679828588919497100
- The message
to:by:do: is sent to an integer
i will get all the computed values one by one
Blocks are Used For Iterators
#(2 4 5 -4 3 -2) collect: [ :each | each abs ]
> #(2 4 5 4 3 2)
- Here the message is sent to the collection itself
- See Lecture on Iterators
Yes ifTrue:ifFalse: is a message!
Weather isRaining
ifTrue: [ self takeMyUmbrella ]
ifFalse: [ self takeMySunglasses ]
- Conceptually
ifTrue:ifFalse: is a message sent to an object: a boolean!
ifTrue:ifFalse: is in fact radically optimized by the compiler
Implementation Note
- Note that the Virtual Machine shortcuts calls to Boolean such as condition for speed reason
- But you can implement your own conditional methods and debug to see that sending a message is dispatching to the right object
- Implement your own control structure such as
siAlors:sinon: (in French) and try it
Summary
- Blocks freeze and control computation
- Basis for
- conditionals
- loops / iterators
- exceptions (see future lectures)
- concurrence
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