On my system (Mozart Engine 1.3.1 (20040617) on Windows XP) the code snippet
behaves as expected by you: the Browser displays "1|3|_".
This is the case even if I include the definition of Filter in the source code.
Maybe a version issue? I'll try this at home on a newer Mozart version later
today.
Wolfgang
> Dear all,
>
> I don't fully understand the behaviour of Filter in concurrent
> programming. List.filter is defined as follows
>
> fun {Filter Xs F}
> case Xs of nil then nil
> [] X|Xr then if {F X} then X|{Filter Xr F}
> else {Filter Xr F} end
> end
> end
>
> I would expect that this definition works even for partly determined
> Xs (i.e. as far as Xs is determined). For example, I expect the code
> below to immediately show in the browser the following: 1|3|_
>
> declare
> Xs = [1 2 3 _ 4]
> Ys
> thread Ys = {Filter Xs IsOdd} end
> {Browse Ys}
>
> However, Filter blocks until Xs is fully determined. Why is this so?
> By contrast, things like Map and ForAll use a very similar case
> statement in their definition (but without an else clause) and they
> work fine for partially determined lists.
>
> Thank you!
>
> Best
> Torsten
>
> BTW: I needed a variant of filter which returns/skips list elements
> as soon as enough information is provided to decided either way, but
> possibly changes the order of list elements:
>
> proc {UnorderedFilter Xs F ?Result}
> P = {NewPort Result}
> in
> {ForAll Xs proc {$ X}
> thread
> if {F X} then {Send P X} end
> end
> end}
> end
>
> --
> Torsten Anders
> Interdisciplinary Centre for Computer Music Research
> University of Plymouth
> Office: +44-1752-233667
> Private: +44-1752-558917
> http://strasheela.sourceforge.net
> http://www.torsten-anders.de
>
>
>
>
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