foldr1 does not replace Data.Foldable ( foldr1 )

[Anton-Latukha]

HNix is a pretty big project. And the migration to relude went smooth, the migration took ~2d of work for everything, and also to debug this issue.

https://github.com/haskell-nix/hnix/blob/cd02c6dd59105ccbfb36218fba3b0bce3c8c77ad/src/Nix/Parser.hs#L607-L608

manyUnaryOp :: MonadPlus f => f (a -> a) -> f (a -> a)
manyUnaryOp f = foldr1 (.) <$> some f

Data.Foldable ( foldr1 ) would be:

If we to use relude foldr1 for that we would receive:

src/Nix/Parser.hs:609:32: error:
    • Couldn't match type ‘[a -> a]’ with ‘a0 -> b0’
      Expected type: f (a0 -> b0)
        Actual type: f [a -> a]
    • In the second argument of ‘(<$>)’, namely ‘some f’
      In the expression: foldr1 (.) <$> some f
      In an equation for ‘manyUnaryOp’:
          manyUnaryOp f = foldr1 (.) <$> some f
    • Relevant bindings include
        f :: f (a -> a) (bound at src/Nix/Parser.hs:609:13)
        manyUnaryOp :: f (a -> a) -> f (a -> a)
          (bound at src/Nix/Parser.hs:609:1)
    |
609 | manyUnaryOp f = foldr1 (.) <$> some f
    |                                ^^^^^^

This is the only thing that was on the Relude side during migration. At first, I've with GHC thought that is some Skolem Existential thing, because was looking from the Relude function side.

But then, nope, Reludes foldr1 asks for a base case, while Foldable one is not.

[chshersh]

I mentioned some challenges in implementing foldr1 in the following comment:

• Add 'foldr1' to the 'Foldable1' typeclass #307 (comment)

In short, changing the type would be nice but the implementation has some challenges and requires a major version bump up.

[Anton-Latukha]

Ok.
Considering this as seen, discussed and closed.