Avoid long tuple definitions in haskell

Question

For my work with hxt I implemented the following function:

-- | Construction of a 8 argument arrow from a 8-ary function. Same
-- implementation as in @Control.Arrow.ArrowList.arr4@.
arr8 :: ArrowList a => (b1 -> b2 -> b3 -> b4 -> b5 -> b6 -> b7 -> b8 -> c)
                -> a (b1, (b2, (b3, (b4, (b5, (b6, (b7, b8))))))) c
arr8 f = arr ( \ ~(x1, ~(x2, ~(x3, ~(x4, ~(x5, ~(x6, ~(x7, x8)))))))
               -> f x1 x2 x3 x4 x5 x6 x7 x8 )

As mentioned in the haddock comment the above function arr8 takes an 8-ary function and returns a 8 argument arrow. I use the function like this: (x1 &&& x2 &&& ... x8) >>> arr8 f whereby x1 to x8 are arrows.

My question: Is there a way to avoid the big tuple definition? Is there a more elegant implementation of arr8?

Info: I used the same code schema as in the function arr4 (see source code of arr4)

Answer 1

This works, though it depends on some quite deep and fragile typeclass magic. It also requires that we change the tuple structure to be a bit more regular. In particular, it should be a type-level linked list preferring (a, (b, (c, ()))) to (a, (b, c)).

{-# LANGUAGE TypeFamilies #-}

import Control.Arrow

-- We need to be able to refer to functions presented as tuples, generically.
-- This is not possible in any straightforward method, so we introduce a type
-- family which recursively computes the desired function type. In particular,
-- we can see that
--
--     Fun (a, (b, ())) r ~ a -> b -> r

type family   Fun h      r :: *
type instance Fun ()     r =  r
type instance Fun (a, h) r =  a -> Fun h r

-- Then, given our newfound function specification syntax we're now in
-- the proper form to give a recursive typeclass definition of what we're
-- after.

class Zup tup where 
  zup :: Fun tup r -> tup -> r

instance Zup () where 
  zup r () = r

-- Note that this recursive instance is simple enough to not require 
-- UndecidableInstances, but normally techniques like this do. That isn't
-- a terrible thing, but if UI is used it's up to the author of the typeclass
-- and its instances to ensure that typechecking terminates.

instance Zup b => Zup (a, b) where 
  zup f ~(a, b) = zup (f a) b

arrTup :: (Arrow a, Zup b) => Fun b c -> a b c
arrTup = arr . zup

And now we can do

> zup (+) (1, (2, ()))
3

> :t arrTup (+)
arrTup (+)
  :: (Num a1, Arrow a, Zup b n, Fun n b c ~ (a1 -> a1 -> a1)) =>
     a b c

> arrTup (+) (1, (2, ()))
3

If you want to define the specific variants, they're all just arrTup.

arr8 
  :: Arrow arr 
  => (a -> b -> c -> d -> e -> f -> g -> h -> r)
  -> arr (a, (b, (c, (d, (e, (f, (g, (h, ())))))))) r
arr8 = arrTup

---

It's finally worth noting that if we define a lazy uncurry

uncurryL :: (a -> b -> c) -> (a, b) -> c
uncurryL f ~(a, b) = f a b

then we can write the recursive branch of Zup in a way that is illustrative to what's going on here

instance Zup b => Zup (a, b) where 
  zup f = uncurryL (zup . f)