Proposal: alpha-rename the type signatures of foldl, foldl', and scanl to be consistent with foldr and scanr
Andreas Abel
andreas.abel at ifi.lmu.de
Sun Oct 14 20:33:51 CEST 2012
+1 to all.
On 14.10.12 6:53 PM, Bas van Dijk wrote:
> +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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>
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--
Andreas Abel <>< Du bist der geliebte Mensch.
Theoretical Computer Science, University of Munich
Oettingenstr. 67, D-80538 Munich, GERMANY
andreas.abel at ifi.lmu.de
http://www2.tcs.ifi.lmu.de/~abel/
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