module Data.Maybe.HT where import Data.Maybe (fromMaybe, ) import Control.Monad (msum, ) {- It was proposed as addition to Data.Maybe and rejected at that time. <http://www.haskell.org/pipermail/libraries/2004-July/002381.html> -} {- | Returns 'Just' if the precondition is fulfilled. -} {-# INLINE toMaybe #-} toMaybe :: Bool -> a -> Maybe a toMaybe False _ = Nothing toMaybe True x = Just x infixl 6 ?-> {- | This is an infix version of 'fmap' for writing 'Data.Bool.HT.select' style expressions using test functions, that produce 'Maybe's. The precedence is chosen to be higher than '(:)', in order to allow: > alternatives default $ > checkForA ?-> (\a -> f a) : > checkForB ?-> (\b -> g b) : > [] The operation is left associative in order to allow to write > checkForA ?-> f ?-> g which is equivalent to > checkForA ?-> g . f due to the functor law. -} (?->) :: Maybe a -> (a -> b) -> Maybe b (?->) = flip fmap alternatives :: a -> [Maybe a] -> a alternatives deflt = fromMaybe deflt . msum