dropWhileRev

[Jonathan Cast]

Henning Thielemann <lemming at henning-thielemann.de> wrote:

> Is this function worth to be added to Data.List?
> 
> 
> {-| Remove the longest suffix of elements satisfying the predicate.
>     In contrast to 'reverse . dropWhile p . reverse'
>     this works for infinite lists, too. -}
> dropWhileRev :: (a -> Bool) -> [a] -> [a]
> dropWhileRev p =
>    foldr (\x xs -> if p x && null xs then [] else x:xs) []

Quick  query: this  function only  delivers partial  lists  from partial
lists;  hence, it  only returns  partial lists  and infinite  lists from
infinite lists

(In particular, it is not true  that, if (exists n. take n (dropWhileRev
p xn) /= take n xn) then (dropWhileRev p xn `isPrefixOf` xn = True).

Prelude> let dropWhileRev p = foldr (\x xs -> if p x && null xs then [] else x:xs) []
Prelude> dropWhileRev (const True) (repeat 'c')
"*** Exception: stack overflow
Prelude> dropWhileRev (\n -> n `mod` 2 == 0) ([1..3]++repeat 2)
[1,2,3*** Exception: stack overflow
Prelude>

What do you mean by ``works for inifinite lists''?

Jon Cast