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

[Bas van Dijk]

+1

There are a few other functions in Data.List that could benefit from
the same treatment:

mapAccumL :: (acc -> x -> (acc, y)) -> acc -> [x] -> (acc, [y])
mapAccumL :: (acc -> a -> (acc, b)) -> acc -> [a] -> (acc, [b])

mapAccumR :: (acc -> x -> (acc, y)) -> acc -> [x] -> (acc, [y])
mapAccumR :: (acc -> a -> (acc, b)) -> acc -> [a] -> (acc, [b])

genericLength :: Num i => [b] -> i
genericLength :: Num i => [a] -> i

genericSplitAt :: Integral i => i -> [b] -> ([b], [b])
genericSplitAt :: Integral i => i -> [a] -> ([a], [a])

genericIndex :: Integral a => [b] -> a -> b
genericIndex :: Integral i => [a] -> i -> a

I'm not sure if we should change mapAccumL/R since 'a' and 'acc' are
maybe too similar.

Bas

On 14 October 2012 16:28, 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.
>
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://www.haskell.org/mailman/listinfo/libraries
>