Proposal: alpha-rename the type signatures of foldl, foldl', and scanl to be consistent with foldr and scanr

[Conal Elliott]

Amen! (+1).

I often have to rewrite the signature of foldl to the suggested version as
a sanity check. I thought it just was my personal mental deficiency.

I prefer "b" to "r".

-- Conall

On Sun, Oct 14, 2012 at 7:28 AM, Gábor Lehel <illissius at gmail.com> wrote:

> Currently we have:
>
>     foldl :: (a -> b -> a) -> a -> [b] -> a
>
>     foldr :: (a -> b -> b) -> b -> [a] -> b
>
> I find this confusing. My brain doesn't do automatic alpha-renaming,
> so I end up thinking that these types are very different because they
> look very different. In fact, they are almost the same.
> Embarrassingly, it took me longer than it took to understand monads,
> GADTs, PolyKinds, and several other things before I realized it!
>
> So I propose that we use 'a' consistently to denote the type of the
> list elements, and 'b' to denote the type of the result:
>
>     foldl :: (b -> a -> b) -> b -> [a] -> b
>
>     foldr :: (a -> b -> b) -> b -> [a] -> b
>
> making it obvious that the only difference is the order of parameters
> to the accumulator.
>
> The total change would be to replace
>
>     Prelude.foldl :: (a -> b -> a) -> a -> [b] -> a
>     Prelude.scanl :: (a -> b -> a) -> a -> [b] -> [a]
>     Data.List.foldl' :: (a -> b -> a) -> a -> [b] -> a
>     Data.Foldable.foldl :: (a -> b -> a) -> a -> t b -> a
>     Data.Foldable.foldl' :: (a -> b -> a) -> a -> t b -> a
>
> with
>
>     Prelude.foldl :: (b -> a -> b) -> b -> [a] -> b
>     Prelude.scanl :: (b -> a -> b) -> b -> [a] -> [b]
>     Data.List.foldl' :: (b -> a -> b) -> b -> [a] -> b
>     Data.Foldable.foldl :: (b -> a -> b) -> b -> t a -> b
>     Data.Foldable.foldl' :: (b -> a -> b) -> b -> t a -> b
>
> I've attached a patch.
>
> Discussion period: 2 weeks
>
> Previously discussed at: http://www.reddit.com/r/haskell/comments/10q2ls/
>
> --
> Your ship was destroyed in a monadic eruption.
>
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