Commit 8dcd4672 authored by Michael Hanus's avatar Michael Hanus
Browse files

Further simple monad operations added

parent e387cd48
......@@ -44,7 +44,8 @@ module Prelude
, PEVAL
, Monad(..)
, Functor(..)
, sequenceM, sequenceM_, mapM, mapM_
, sequenceM, sequenceM_, mapM, mapM_, foldM, liftM, forM, forM_
, unlessM, whenM
#ifdef __PAKCS__
, (=:<<=), letrec
#endif
......@@ -1807,7 +1808,7 @@ instance Monad [] where
fail _ = []
----------------------------------------------------------------------------
-- Some further useful monad operations which might be later generalized
-- Some useful monad operations which might be later generalized
-- or moved into some other base module.
--- Evaluates a sequence of monadic actions and collects all results in a list.
......@@ -1831,4 +1832,42 @@ mapM f = sequenceM . map f
mapM_ :: Monad m => (a -> m _) -> [a] -> m ()
mapM_ f = sequenceM_ . map f
--- Folds a list of elements using a binary monadic action and a value
--- for the empty list.
foldM :: Monad m => (a -> b -> m a) -> a -> [b] -> m a
foldM _ z [] = return z
foldM f z (x:xs) = f z x >>= \z' -> foldM f z' xs
--- Apply a pure function to the result of a monadic action.
liftM :: Monad m => (a -> b) -> m a -> m b
liftM f m = m >>= return . f
--- Like `mapM`, but with flipped arguments.
---
--- This can be useful if the definition of the function is longer
--- than those of the list, like in
---
--- forM [1..10] $ \n -> do
--- ...
forM :: Monad m => [a] -> (a -> m b) -> m [b]
forM xs f = mapM f xs
--- Like `mapM_`, but with flipped arguments.
---
--- This can be useful if the definition of the function is longer
--- than those of the list, like in
---
--- forM_ [1..10] $ \n -> do
--- ...
forM_ :: Monad m => [a] -> (a -> m b) -> m ()
forM_ xs f = mapM_ f xs
--- Performs a monadic action unless the condition is met.
unlessM :: Monad m => Bool -> m () -> m ()
unlessM p act = if p then return () else act
--- Performs a monadic action when the condition is met.
whenM :: Monad m => Bool -> m () -> m ()
whenM p act = if p then act else return ()
----------------------------------------------------------------------------
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