Re: [Haskell-cafe] Can this be made to fuse?

A simple composition of a pure [Type] -> Type function, fmap'ped and Control.Monad.sequence is not going to help, is it? At least the sequence might fuse with a producer, not sure if the fmap then gets in the way to fuse with the pure function though. 22.09.2022 20:14:20 David Feuer :

The recursion is the first barrier. The whole thing ends up a loop breaker. Using `fix` fixes that, but it still doesn't fuse for some reason. After I sent this, I realized that foldl' is really the wrong thing, since that builds up a large Q action that, when run, produces the expression. The nicer thing is to use foldM within Q, and wrap mfix around that. I'm still not seeing fusion with that, and I wonder if that's because of the complexity of the Q type with its higher-rank constrained polymorphism and such.

On Thu, Sep 22, 2022, 1:56 PM Joachim Breitner wrote:

...

Hi,

Am Sonntag, dem 11.09.2022 um 19:05 -0400 schrieb David Feuer:

...

The template-haskell package has functions `tupE` and `tupP` for building tuple expressions and patterns, respectively, but nothing really similar for building tuple types. I came up with the version below the other day:

-- | Given a list of types, produce the type of a tuple of -- those types. This is analogous to 'tupE' and 'tupP'. -- -- @ -- tupT [[t|Int|], [t|Char|], [t|Bool]] = [t| (Int, Char, Bool) |] -- @ tupT :: [Q Type] -> Q Type tupT ts = n `seq` res    where      -- We build the expression with a thunk inside that will be filled in with      -- the length of the list once that's been determined. This works      -- efficiently (in one pass) because TH.Type is rather lazy.      (res, n) = foldl' (\(acc, !k) ty -> ([t| $acc $ty |], k + 1))                        (tupleT n, 0)                        ts

I strongly suspect this is quite fast enough in practice, but it's a bit annoying that it won't participate in list fusion; tupT (map f xs) will (lazily) generate an intermediate list. I wasn't able to convince GHC to fuse it, short of a custom rewrite rule or two (tupT (build f) = ..., tupT (augment f r = ...). Does anyone know if it's possible?

Can you say why it would not fuse? It seems it could, if tupT inlines, and then you have foldl' applied to (map f xs), and at this point I would hope that fusion kicks in.

Cheers, Joachim

-- Joachim Breitner   mail@joachim-breitner.de   http://www.joachim-breitner.de/

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.