Prelude function suggestions
Henning Thielemann
iakd0 at clusterf.urz.uni-halle.de
Mon Jul 26 06:34:36 EDT 2004
There are some functions that I regularly use which are so general that I
wonder if they should be in the Prelude. Maybe I have overlooked them or
some generalizations of them. Here are the ones that I most frequently
use:
{- | Compositional power of a function,
i.e. apply the function n times to a value. -}
nest :: Int -> (a -> a) -> a -> a
nest 0 f x = x
nest n f x = f (nest (n-1) f x)
{- | Split the list at the occurrences of a separator into sub-list.
This is a generalization of 'words'. -}
chop :: (a -> Bool) -> [a] -> [[a]]
chop p s =
let (l, s') = break p s
in l : case s' of
[] -> []
(_:rest) -> chop p rest
{- | Returns 'Just' if the precondition is fulfilled. -}
toMaybe :: Bool -> a -> Maybe a
toMaybe False _ = Nothing
toMaybe True x = Just x
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