Messages: Composition and Precedence
Damien Cassou, Stéphane Ducasse and Luc Fabresse
http://stephane.ducasse.free.fr
Composition: from Left to Right!
What happens when we have two messages of the same kind?
- Execution from left to right
1000 factorial class name
> 'LargePositiveInteger'
is equivalent to
(((1000 factorial) class) name)
- Ease the composition of messages
Complete Message Precedence
- (Msg) > Unary > Binary > Keywords
- From left to right
Precedence Example
2 + 3 squared
> 2 + 9
> 11
- unary (squared) first
- then binary (
+)
Precedence Example
2 raisedTo: 3 + 2
> 2 raisedTo: 5
> 32
- binary (
+) first
- then keyword-based (
raisedTo:)
Precedence Example
Color gray - Color white = Color black
> aGray - aWhite = aBlack
> aBlack = aBlack
> true
- unary messages
- then binary from left to right
Precedence Example
1 class maxVal + 1
> 1073741824
1 class
> SmallInteger
1 class maxVal
> 1073741823
1 class maxVal + 1
> 1073741824
(1 class maxVal + 1) class
> LargePositiveInteger
Parentheses take Precedence!
0@0 extent: 100@100 bottomRight
> Message not understood
> 100 does not understand bottomRight
Should use ()
(0@0 extent: 100@100) bottomRight
> (aPoint extent: anotherPoint) bottomRight
> aRectangle bottomRight
> 100@100
The Price for Simplicity
Only messages:
+
- is a message, no precedence
- can be redefined in domain classes
- Simple
- One limit: no mathematical precedence
No Mathematical Precedence
3 + 2 * 10
> 5 * 10
> 50
- should be rewritten using parentheses
3 + (2 * 10)
> 3 + 20
> 23
No Mathematical Precedence
1/3 + 2/3
> 7/3 /3
> 7/9
- should be rewritten using parentheses
(1/3) + (2/3)
> 1
Summary
- Three kinds of messages: unary, binary and keywords
- (...) > unary > binary > keywords
- Then from left to right
- There is no mathematical precedence because mathematical operations are plain messages
- Arguments are placed inside message structure:
- 2 between: 0 and: 5 (the message is
between:and:)
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