change of strictness in zipWith

[Roman Cheplyaka]

This is a big deal. That's a very common pattern.

On 02/01/15 11:50, Henning Thielemann wrote:
> 
> $ ghci-7.8.4 -Wall
> Prelude> zipWith (==) "" undefined
> []
> 
> $ ghci-7.10.0.20141227 -Wall
> Prelude> zipWith (==) "" undefined
> *** Exception: Prelude.undefined
> 
> 
> I found the difference because my mapAdjacent function uses 'tail' in
> the second argument of 'zipWith':
> 
> mapAdjacent :: (a -> a -> b) -> [a] -> [b]
> mapAdjacent f xs = zipWith f xs (tail xs)
> 
> 
> I get:
> 
> $ ghci-7.10.0.20141227 -Wall
> *Prelude> Data.List.HT.mapAdjacent (==) ([] :: [Int])
> *** Exception: Prelude.tail: empty list
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