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/