[squeak-dev] Escaping from loops in scripts

Tobias Pape Das.Linux at gmx.de
Sun Dec 9 22:16:21 UTC 2018


> On 09.12.2018, at 21:22, Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com> wrote:
> 
> Hi Tobias,
> I don't want to pay the burden of making own class/methods, I just want to write a dumb and dirty script.

Makes sense…
I probably would do nested blocks, but that's equally inelegant.

> 
> But to answer to myself, what I asked is already in the image:
> 
> BlockClosure>>valueWithExit 
>       self value: [ ^nil ]
> 
> It would be more convenient with a return value, but for now it's ok.

That's a very interesting thing find, thank you! 
:)

Best regards
	-Tobias

> 
> Le dim. 9 déc. 2018 à 14:08, Tobias Pape <Das.Linux at gmx.de> a écrit :
> Hi Nicolas,
> 
> 
> > On 09.12.2018, at 11:37, Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com> wrote:
> > 
> > Hi all,
> > when writing scripts, I often need the ability to interrupt a loop.
> > This is the traditional break instruction of C/Python or more advanced exit of FORTRAN/ADA (we can tell from which nexted loop we exit).
> > 
> > I could use exceptions, but then handling exit from nested loops is tricky (see below).
> > One possible way to do it in Smalltalk is to use blocks, and interrupt with non local return.
> > 
> 
> 
> My way would be to use at most one method per loop and have an exit block then, like so
> 
> 
> A>>run: x
>   |y z |
>   y = 0.
>   z := x
>   [z < 10] whileTrue: 
>      [y := self foobar: z ifExit: [ ^y].
>       z := z + 1]
>   ^ y
> 
> A>> foobar: x ifExit: aBlock
> 
>   | y |
>   y := x.
>   [y isOdd] whileTrue:
>     [y := y + 2.
>     " somehting something"
>     y > 3 ifTrue: aBlock]
>   ^ y 
> 
> Best regards
>         -Tobias
> 
> > ref http://lists.gnu.org/archive/html/help-smalltalk/2008-06/msg00077.html
> > 
> > Object>>escape: aBlock
> >     "Give the ability to exit execution of aBlock by returning control to sender.
> >     aBlock will take an argument, which is a door for returning control.
> >     This is useful for breaking a loop, for example:
> >      self escape: [:exit | someCollection do: [:e | e fullfilSomeCondition ifTrue: [exit value]. e doSometing]]"
> >     aBlock value: [^self]
> > 
> > | sum |
> > sum := 0.
> > self escape: [:exitOuterLoop | someCollection do: [:e |
> >     self escape: [:exitInnerLoop | someOtherCollection do: [:e2 |
> >         e2 > e ifTrue: [exitOuterLoop value].
> >         e2 = e ifTrue: [exitInnerLoop value].
> >         sum := sum + e]]]].
> > ^sum
> > 
> > We can also use the escape: inside the loop to avoid chained ifTrue:ifFalse:
> > but it's then convenient to let escape: return a value:
> > 
> > Object>>escape: aBlock
> >     ^aBlock value: [:result | ^result]
> > 
> > aCollection collect: [:e |
> >     e escape: [:return |
> >         e < 0 ifTrue: [return value: e].
> >         e > 20 ifTrue: [return value: 401 ln].
> >        (e squared + 1) ln]]
> > 
> > ref https://stackoverflow.com/questions/7547750/smalltalk-block-can-i-explicitly-set-the-returning-value-and-stop-executing-th/11532045#11532045
> > 
> > At that time, I found that amusing, now I find it useful.
> > I don't see such support in trunk image, could we add it?
> > Do you think of a better name?
> > (not setjmp: please)
> > 
> > Unless you come with better alternatives... One thing that is questionable is that the receiver of escape: is void (escape: is a utility).
> > 
> > There is https://stackoverflow.com/questions/52683795/gnu-smalltalk-break-from-whiletrue-loop-without-return/52702174#52702174 
> > It start looking like FORTRAN/ADA exit instruction with explicit naming of loop but I don't like it better.
> > 
> > Maybe sending the message to the block itself sounds less arbitrary.
> > 
> > BlockClosure>>handleExit
> >     ^self value: [:result | ^result]
> > 
> > | sum |
> > sum := 0.
> > [:exitOuterLoop | someCollection do: [:e |
> >      [:exitInnerLoop | someOtherCollection do: [:e2 |
> >         e2 > e ifTrue: [exitOuterLoop value: nil].
> >         e2 = e ifTrue: [exitInnerLoop value: nil].
> >         sum := sum + e]] handleExit ]] handleExit.
> > ^sum
> > 
> > Though, I don't find such post-fixing satisfactory: exitOuterLoop and handleExit are too far from each other... Especially in scripts that tend to be longer than ordinary methods (we don't want to factor every quick and dirty procedure into proper classes/methods when there is no reuse objective or when they are too specific).
> > 
> 
> 
> 



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