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

[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 <mail at joachim-breitner.de>
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 at joachim-breitner.de
>   http://www.joachim-breitner.de/
>
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