1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
|
{- monadic stuff
-
- Copyright 2010-2012 Joey Hess <joey@kitenet.net>
-
- Licensed under the GNU GPL version 3 or higher.
-}
module Utility.Monad where
import Data.Maybe
import Control.Monad
{- Return the first value from a list, if any, satisfying the given
- predicate -}
firstM :: Monad m => (a -> m Bool) -> [a] -> m (Maybe a)
firstM _ [] = return Nothing
firstM p (x:xs) = ifM (p x) (return $ Just x , firstM p xs)
{- Runs the action on values from the list until it succeeds, returning
- its result. -}
getM :: Monad m => (a -> m (Maybe b)) -> [a] -> m (Maybe b)
getM _ [] = return Nothing
getM p (x:xs) = maybe (getM p xs) (return . Just) =<< p x
{- Returns true if any value in the list satisfies the predicate,
- stopping once one is found. -}
anyM :: Monad m => (a -> m Bool) -> [a] -> m Bool
anyM p = liftM isJust . firstM p
allM :: Monad m => (a -> m Bool) -> [a] -> m Bool
allM _ [] = return True
allM p (x:xs) = p x <&&> allM p xs
{- Runs an action on values from a list until it succeeds. -}
untilTrue :: Monad m => [a] -> (a -> m Bool) -> m Bool
untilTrue = flip anyM
{- if with a monadic conditional. -}
ifM :: Monad m => m Bool -> (m a, m a) -> m a
ifM cond (thenclause, elseclause) = do
c <- cond
if c then thenclause else elseclause
{- short-circuiting monadic || -}
(<||>) :: Monad m => m Bool -> m Bool -> m Bool
ma <||> mb = ifM ma ( return True , mb )
{- short-circuiting monadic && -}
(<&&>) :: Monad m => m Bool -> m Bool -> m Bool
ma <&&> mb = ifM ma ( mb , return False )
{- Same fixity as && and || -}
infixr 3 <&&>
infixr 2 <||>
{- Left-to-right Kleisli composition with a pure left/right hand side. -}
(*>=>) :: Monad m => (a -> b) -> (b -> m c) -> (a -> m c)
f *>=> g = return . f >=> g
(>=*>) :: Monad m => (a -> m b) -> (b -> c) -> (a -> m c)
f >=*> g = f >=> return . g
{- Same fixity as >=> and <=< -}
infixr 1 *>=>, >=*>
{- Runs an action, passing its value to an observer before returning it. -}
observe :: Monad m => (a -> m b) -> m a -> m a
observe observer a = do
r <- a
_ <- observer r
return r
{- b `after` a runs first a, then b, and returns the value of a -}
after :: Monad m => m b -> m a -> m a
after = observe . const
{- do nothing -}
noop :: Monad m => m ()
noop = return ()
|
