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Diffstat (limited to 'ghc/lib/std/Monad.lhs')
| -rw-r--r-- | ghc/lib/std/Monad.lhs | 118 |
1 files changed, 118 insertions, 0 deletions
diff --git a/ghc/lib/std/Monad.lhs b/ghc/lib/std/Monad.lhs new file mode 100644 index 0000000000..2be1dba439 --- /dev/null +++ b/ghc/lib/std/Monad.lhs @@ -0,0 +1,118 @@ +% +% (c) The AQUA Project, Glasgow University, 1994-1996 +% +\section[Monad]{Module @Monad@} + +\begin{code} +{-# OPTIONS -fno-implicit-prelude #-} + +module Monad ( + Functor(..), + Monad(..), MonadZero(..), MonadPlus(..), + + -- Prelude monad functions + accumulate, sequence, + mapM, mapM_, guard, filter, concat, applyM, + + -- Standard Monad interface: + join, -- :: (Monad m) => m (m a) -> m a + mapAndUnzipM, -- :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c]) + zipWithM, -- :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c] + zipWithM_, -- :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m () + foldM, -- :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a + when, -- :: (Monad m) => Bool -> m () -> m () + unless, -- :: (Monad m) => Bool -> m () -> m () + ap, -- :: (Monad m) => (m (a -> b)) -> (m a) -> m b + liftM, liftM2, + liftM3, liftM4, + liftM5 + ) where + +import PrelList +import PrelTup +import PrelBase +\end{code} + +%********************************************************* +%* * +\subsection{Functions mandated by the Prelude} +%* * +%********************************************************* + +\begin{code} +accumulate :: Monad m => [m a] -> m [a] +accumulate [] = return [] +accumulate (m:ms) = do { x <- m; xs <- accumulate ms; return (x:xs) } + +sequence :: Monad m => [m a] -> m () +sequence = foldr (>>) (return ()) + +mapM :: Monad m => (a -> m b) -> [a] -> m [b] +mapM f as = accumulate (map f as) + +mapM_ :: Monad m => (a -> m b) -> [a] -> m () +mapM_ f as = sequence (map f as) + +guard :: MonadZero m => Bool -> m () +guard p = if p then return () else zero + +-- This subsumes the list-based filter function. + +filter :: MonadZero m => (a -> Bool) -> m a -> m a +filter p = applyM (\x -> if p x then return x else zero) + +-- This subsumes the list-based concat function. + +concat :: MonadPlus m => [m a] -> m a +concat = foldr (++) zero + +applyM :: Monad m => (a -> m b) -> m a -> m b +applyM f x = x >>= f +\end{code} + + +%********************************************************* +%* * +\subsection{Other monad functions} +%* * +%********************************************************* + +\begin{code} +join :: (Monad m) => m (m a) -> m a +join x = x >>= id + +mapAndUnzipM :: (Monad m) => (a -> m (b,c)) -> [a] -> m ([b], [c]) +mapAndUnzipM f xs = accumulate (map f xs) >>= return . unzip + +zipWithM :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m [c] +zipWithM f xs ys = accumulate (zipWith f xs ys) + +zipWithM_ :: (Monad m) => (a -> b -> m c) -> [a] -> [b] -> m () +zipWithM_ f xs ys = sequence (zipWith f xs ys) + +foldM :: (Monad m) => (a -> b -> m a) -> a -> [b] -> m a +foldM f a [] = return a +foldM f a (x:xs) = f a x >>= \fax -> foldM f fax xs + +unless :: (Monad m) => Bool -> m () -> m () +unless p s = if p then return () else s + +when :: (Monad m) => Bool -> m () -> m () +when p s = if p then s else return () + +ap :: (Monad m) => m (a->b) -> m a -> m b +ap = liftM2 ($) + +liftM :: (Monad m) => (a1 -> r) -> m a1 -> m r +liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r +liftM3 :: (Monad m) => (a1 -> a2 -> a3 -> r) -> m a1 -> m a2 -> m a3 -> m r +liftM4 :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m r +liftM5 :: (Monad m) => (a1 -> a2 -> a3 -> a4 -> a5 -> r) -> m a1 -> m a2 -> m a3 -> m a4 -> m a5 -> m r + +liftM f m1 = do { x1 <- m1; return (f x1) } +liftM2 f m1 m2 = do { x1 <- m1; x2 <- m2; return (f x1 x2) } +liftM3 f m1 m2 m3 = do { x1 <- m1; x2 <- m2; x3 <- m3; return (f x1 x2 x3) } +liftM4 f m1 m2 m3 m4 = do { x1 <- m1; x2 <- m2; x3 <- m3; x4 <- m4; return (f x1 x2 x3 x4) } +liftM5 f m1 m2 m3 m4 m5 = do { x1 <- m1; x2 <- m2; x3 <- m3; x4 <- m4; x5 <- m5; return (f x1 x2 x3 x4 x5) } + +\end{code} |