Proposal: alpha-rename the type signatures of foldl, foldl', and scanl to be consistent with foldr and scanr
Bas van Dijk
v.dijk.bas at gmail.com
Sun Oct 14 18:53:57 CEST 2012
+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.
>
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