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

On Sun, Oct 14, 2012 at 8:47 PM, Dan Burton wrote:

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

'r' is the version I originally used myself (see reddit). But then you have to change the foldrs too, and quite possibly a lot of other functions, and get dragged into a big discussion over when is 'r' a better mnemonic than 'b' and when is it not. Simply swapping 'a' and 'b' in the left folds captures most of the benefit for least cost. Same goes for most of the other suggestions I have seen (I don't find the existing signatures actively confusing, merely suboptimal), but if sentiment is overwhelmingly in favor of making further changes I can be swayed. For now I wanted to propose a minimal change with the best chance of attracting a broad consensus.

On Sun, Oct 14, 2012 at 12:33 PM, Andreas Abel 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 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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Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY

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-- Your ship was destroyed in a monadic eruption.