On Thu, 29 Jul 2004, ariep wrote:
{- | 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)
nest n f x = iterate f x !! n
That might render 'nest' somewhat superfluous.
What about: nest n f = foldl (.) id (replicate n f) :-) I'm aware of such transcriptions, but most commonly I use the partial application (nest n f) which can't be provided by them. If 'nest' (or maybe renamed to 'compPower') will be added, anyway, I think it should go where (.) is defined.
{- | 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
I like 'chop'. It belongs in Data.List, I'd say.
Nice that you like it. :-) Will the text processing functions like 'words' and 'lines' go to a module separate from Data.List in future?
{- | Returns 'Just' if the precondition is fulfilled. -} toMaybe :: Bool -> a -> Maybe a toMaybe False _ = Nothing toMaybe True x = Just x
Could you give an example to show what makes 'toMaybe' a particularly useful function?
Say, you have some stop condition that you want to use in connection with unfoldr: unfoldr (\n -> toMaybe (n<10) (n,n+1)) 0 Or say you want to implement a function f :: Set (Maybe a) -> Maybe (Set a) where the result is Nothing if any element of the Set is Nothing, and Just the set containing the Just values otherwise: f s = toMaybe (not (Nothing `elementOf` s)) (mapSet fromJust s)