Re: [Haskell-cafe] Stream fusion and span/break/group/init/tails

Ah, good point. On Thu, Apr 25, 2013 at 1:06 AM, Dan Doel wrote:

Presumably concat also has to use skip, for the same reason as filter. Otherwise it has to recursively process the outer stream until it gets to a non-empty inner stream, which breaks the rule that only the final consumer is recursive.

concat [[1,2,3],[4,5],[],[6,7]]

probably looks something like:

Yield 1, Yield 2, Yield 3, Skip, Yield 4, Yield 5, Skip, Skip, Yield 6, Yield 7, Skip, Done

-- Dan

On Wed, Apr 24, 2013 at 6:52 PM, Gábor Lehel wrote:

...

On Wed, Apr 24, 2013 at 7:56 PM, Bryan O'Sullivan wrote:

...

On Wed, Apr 24, 2013 at 10:47 AM, Duncan Coutts < duncan.coutts@googlemail.com> wrote:

...

I address it briefly in my thesis [1], Section 4.8.2. I think it's a fundamental limitation of stream fusion.

See also concat, where the naive fusion-based implementation has quadratic performance:

concat :: [Text] -> Text concat txts = unstream (Stream.concat (List.map stream txts))

I've never figured out how to implement this with sensible characteristics within the fusion framework.

If you could "solve" concat, might that also lead to be being able to do without the Skip constructor? Skip was added explicitly to be able to efficiently handle things like filter (without it the Step datatype is exactly the base functor for lists), but if concat "works", then filter p can be expressed as concat . map (\x -> if (p x) then [x] else []). Of course, presumably filter isn't the only function that requires Skip, I don't know what the others are. (Also of course "solve" and "works" are intentionally fuzzy, with the point being that I don't know if "solving" concat implies that the desirable properties would survive composition in the suggested manner.)

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