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

Seconded. This has been on my list of small time annoyances for years.

On Sun, Oct 14, 2012 at 10:28 AM, Gábor Lehel <[email protected]> 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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