Monoid Comprehensions

monoidComprehensions.hs

{-# LANGUAGE MonadComprehensions, RebindableSyntax #-}

import Prelude hiding (return, (>>=), (>>), guard)

return :: a -> a
return = id
(>>=) :: (Foldable f, Monoid m) => f a -> (a -> m) -> m
(>>=) = flip foldMap

guard :: Bool -> Bool
guard = id
(>>) :: Monoid m => Bool -> m -> m
(>>) True m = m
(>>) False _ = mempty

query 
  :: (Monoid m, Foldable f1, Foldable f2)
  => (a -> b -> Bool) -> (a -> b -> m) -> f1 a -> f2 b -> m
query f g xs ys = [ g x y | x <- xs, y <- ys, f x y ]