haskweb/Shared.hs

module Shared (
    defRunMaybeT, (<$$>), MaybeT(..), ifF, ifM, (<:), (<->)
) where

import Data.Maybe
import Control.Monad.Trans.Maybe

defRunMaybeT :: Monad m => a -> MaybeT m a -> m a
defRunMaybeT defValue m = fromMaybe defValue <$> runMaybeT m

(<$$>) :: (Functor f, Functor g) => (a -> b) -> f (g a) -> f (g b)
(<$$>) = fmap . fmap

ifF :: Bool -> a -> a -> a
ifF cond x y = if cond then x else y

ifF' :: a -> a -> Bool -> a
ifF' x y cond = if cond then x else y

ifM :: Monad m => m Bool -> m a -> m a -> m a
ifM cond m1 m2 = cond >>= ifF' m1 m2

(<:) :: Monad m => m Bool -> m a -> m a -> m a
(<:) = ifM

(<->) :: (a -> b) -> a -> b
(<->) = ($)