A little puzzle

>Hello all,
>
>I'm finding myself unable to implement something seemingly simple.  Here
>is the puzzle:
>
>I want an iterator factory which, given a specific collection, produces
>a block taking two arguments, also blocks. One (the doBlock) tells what
>should be done for each element of the collection, and the other (the
>whileBlock) is a test allowing to abort the whole operation.
>
>So, something like this:
>
>Puzzle>>blockIterating: aCollection
>
> ^ [:doBlock :whileBlock |
> aCollection do: [:i |
> (whileBlock value: i) ifFalse: [^ self].
> doBlock value: i]].
>
>
>Then I could do things like the following:
>
>
>| block |
>
>block := Puzzle new blockIterating: (1 to: 5).
>
>block value: [:p | Transcript show: p; cr] value: [:p | p < 3]
>
>
>
>But the above fails with a 'Block cannot return' (that's the [^ self]
>block in the #blockIterating: method). I have attached the code; just do
>"Puzzle new fail".
>
>I can't find a workaround.
>
>How should I proceed to get a working iterator block ?
>
>Stef

> Returning from a block means returning from the method containing that
> block. (There's no syntax for block return in Smalltalk, because you'd
> have to be able to tell which block you want to return from to make that
> be of any use, but blocks are anonymous methods. If you need a named
> method, then use one.).
> You get that error, because you've already returned from the method (the
> block itself was returned), and it's not possible to return twice from the
> same context.
> One easy way to make it work is to send a message from inside the block,
> because then you'll have a separate context to return from.
> E.g.:
>
> Puzzle >> #blockIterating: aCollection
>
>   ^[ :doBlock :whileBlock |
>   self iterate: aCollection do: doBlock while: whileBlock ].
>
> Puzzle >> #iterate: aCollection do: doBlock while: whileBlock
>
>   aCollection do: [ :each |
>   (whileBlock value: each) ifFalse: [ ^self ].
>   doBlock value: each ]
>
> Levente
>
> On Thu, 14 May 2015, Stéphane Rollandin wrote:
>
>> Hello all,
>>
>> I'm finding myself unable to implement something seemingly simple.  Here is
>> the puzzle:
>>
>> I want an iterator factory which, given a specific collection, produces a
>> block taking two arguments, also blocks. One (the doBlock) tells what should
>> be done for each element of the collection, and the other (the whileBlock) is
>> a test allowing to abort the whole operation.
>>
>> So, something like this:
>>
>> Puzzle>>blockIterating: aCollection
>>
>> ^ [:doBlock :whileBlock |
>> aCollection do: [:i |
>> (whileBlock value: i) ifFalse: [^ self].
>> doBlock value: i]].
>>
>>
>> Then I could do things like the following:
>>
>>
>> | block |
>>
>> block := Puzzle new blockIterating: (1 to: 5).
>>
>> block value: [:p | Transcript show: p; cr] value: [:p | p < 3]
>>
>>
>>
>> But the above fails with a 'Block cannot return' (that's the [^ self] block
>> in the #blockIterating: method). I have attached the code; just do "Puzzle
>> new fail".
>>
>> I can't find a workaround.
>>
>> How should I proceed to get a working iterator block ?
>>
>> Stef
>>
>
>
>
>

> Ah, so that's the bottom line. Thanks a lot ! I'm now trying to wrap my
> head around these concepts...
>
> It looks like I could also get the return block as an outside argument,
> like this:
>
>
> Puzzle>>blockIterating: aCollection
>
> ^ [:returnBlock |
> [:doBlock :whileBlock |
> aCollection do: [:i |
> (whileBlock value: i) ifFalse: returnBlock.
> doBlock value: i]]].
>
>
> and then use it as follow:
>
>
> | block |
>
> block := (Puzzle new blockIterating: (1 to: 5)) value: [ ^self].
>
> block value: [:p | Transcript show: p; cr] value: [:p | p < 3]
>
>
> A bit hard to read but it seems to work. Does that make sense ?

> Hello all,
>
> I'm finding myself unable to implement something seemingly simple.  
> Here is the puzzle:
>
> I want an iterator factory which, given a specific collection,  
> produces a block taking two arguments, also blocks. One (the doBlock)  
> tells what should be done for each element of the collection, and the  
> other (the whileBlock) is a test allowing to abort the whole  
> operation.
>
> So, something like this:
>
> Puzzle>>blockIterating: aCollection
>
> ^ [:doBlock :whileBlock |
> aCollection do: [:i |
> (whileBlock value: i) ifFalse: [^ self].
> doBlock value: i]].
>
>
> Then I could do things like the following:
>
>
> | block |
>
> block := Puzzle new blockIterating: (1 to: 5).
>
> block value: [:p | Transcript show: p; cr] value: [:p | p < 3]
>
>
>
> But the above fails with a 'Block cannot return' (that's the [^ self]  
> block in the #blockIterating: method). I have attached the code; just  
> do "Puzzle new fail".
>
> I can't find a workaround.
>
> How should I proceed to get a working iterator block ?
>
> Stef