[Haskell-beginners] Exercise of "Programming with Arrows"

[Thiago Negri]

Okay, I solved the exact test case the paper proposed.

Now I've hit another problem:

> -- -- -- stupid loop
> -- `runSP loop_swap [1..10]` works
> loop_swap :: SP a a
> loop_swap = let swap (a, b) = (b, a) in loop (arr swap)

> -- loop_bufid is just a "delayed id"
> -- `runSP loop_bufid [1..10]` doesn't works :(
> loop_bufid :: SP a a
> loop_bufid = loop (Get (\a -> Get (\b -> Put a (Put b id))))

Updated gist: https://gist.github.com/thiago-negri/2e541a9f9762c727bdd4

At least I'm doing progress. :)



2013/10/14 Thiago Negri <evohunz at gmail.com>

> Thanks for the new reply.
>
> I've fixed the "queue" not being a real queue (see the last gist I've sent
> to the list [1]).
>
> The problem I was having in defining "first" was in my understanding of
> the execution of the arrow.
> I didn't realize that (>>>) is just the composition function from Category
> class.
> I tought that only the last output would be received by the second arrow
> of (>>>), and it would continue the stream processing from there.
> I was really lost.
> Now I see that the correct thing to do is to fit a no-op on the second
> stream because outputs will be delivered in pairs to the composed arrow.
> Sort of a BalanceLine/Merge that I've learned in my COBOL years.
> I will run your test cases as soon as I get home.
>
> By the way, I couldn't figure out a way to define an ArrowLoop instance
> that "runSP (loop (arr swap))) [1..10]" doesn't evaluate to bottom (loops
> forever).
> I don't have much time to this, but I'm trying to solve it mentally when I
> feel in the mood and typing some code on a 30-min/day basis.
> The hackage package that defines a Stream Processor as explained by Hughes
> uses a different definition for SP data type and has the same problem when
> evaluating "swap" in a loop (it throws an error) [2].
> I guess this problem is harder than Hughes expected, as I find it more
> difficult to solve than the tricky "first" definition.
> Or, I may be missing something huge.
> (I still want to solve it by myself, but tips are welcome.)
>
> Thanks,
> Thiago.
>
> [1] https://gist.github.com/thiago-negri/2e541a9f9762c727bdd4
> [2] http://hackage.haskell.org/package/streamproc
>
>
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