[Pharo-users] Enhancing SequenceableCollection>>combinationsAt:in:after:do:

Nov. 2, 2011

      In Pharo 1.3, SequenceableCollection>>combinations:atATimeDo: will,
for all combinations, send the same collection instance as argument
for the block. Therefore, if you use this collection itself, you will
be surprised:

|answer|
answer := OrderedCollection new.
#(a b c) combinations: 2 atATimeDo: [:each | answer add: each].
answer

Here I expect to end up with a collection looking like this:
1:  #(#a #b)
2:  #(#a #c)
3:  #(#b #c)

But I end up with:
1:  #(#c #c)
2:  #(#c #c)
3:  #(#c #c)

One fix is to change the client code to copy the argument for the block:
|answer|
answer := OrderedCollection new.
#(a b c) combinations: 2 atATimeDo: [:each | answer add: each copy].
answer

Another solution is enhancing
SequenceableCollection>>combinationsAt:in:after:do: to copy the
collection:
nn + 1 to: self size do: [ :index |
		aCollection at: jj put: (self at: index).
		jj = aCollection size
			ifTrue: [ aBlock value: aCollection copy ]
			ifFalse: [
				self
					combinationsAt: jj + 1
					in: aCollection
					after: index
					do: aBlock ] ]

Kind regards
Runar Jordahl
blog.epigent.com