Proposal: alpha-rename the type signatures of foldl, foldl', and scanl to be consistent with foldr and scanr
Dan Burton
danburton.email at gmail.com
Sun Oct 14 20:47:46 CEST 2012
At the risk of useless bikeshedding... might I suggest "r" as a
mnemonic for "result"?
foldl :: (a -> r -> r) -> r -> [a] -> r
foldr :: (r -> a -> r) -> r -> [a] -> r
-- Dan Burton
On Sun, Oct 14, 2012 at 12:33 PM, Andreas Abel <andreas.abel at ifi.lmu.de>wrote:
> +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/<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/ <http://www2.tcs.ifi.lmu.de/~abel/>
>
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