Safe Haskell	Safe-Inferred
Language	Haskell98

Control.Error

Contents

Re-exports

Description

Import this module in your code to access the entire library's functionality:

import Control.Error

This module exports the entire library as well as useful exports from other standard error-handling libraries:

Control.Error.Safe: Generalizes the safe library, including Either, EitherT, and MonadPlus variations on total functions
Control.Error.Script: Support for simple scripts that catch all errors and transform them to Text
Control.Error.Util: Utility functions and conversions between common error-handling types
Control.Monad.Trans.Except: The ExceptT monad transformer
Control.Monad.Trans.Maybe: The MaybeT monad transformer
Data.Either: Either utility functions
Data.EitherR: throw and catch functions, and their corresponding "success" monads
Data.Maybe: Maybe utility functions
Safe: Total versions of partial Prelude functions

This module does not re-export partial functions from other libraries.

Synopsis

module Control.Error.Safe
module Control.Error.Script
module Control.Error.Util
newtype ExceptT e (m :: Type -> Type) a = ExceptT (m (Either e a))
throwE :: forall (m :: Type -> Type) e a. Monad m => e -> ExceptT e m a
runExceptT :: ExceptT e m a -> m (Either e a)
mapExceptT :: (m (Either e a) -> n (Either e' b)) -> ExceptT e m a -> ExceptT e' n b
withExceptT :: forall (m :: Type -> Type) e e' a. Functor m => (e -> e') -> ExceptT e m a -> ExceptT e' m a
catchE :: forall (m :: Type -> Type) e a e'. Monad m => ExceptT e m a -> (e -> ExceptT e' m a) -> ExceptT e' m a
newtype MaybeT (m :: Type -> Type) a = MaybeT {
- runMaybeT :: m (Maybe a)
}
liftCallCC :: CallCC m (Maybe a) (Maybe b) -> CallCC (MaybeT m) a b
liftCatch :: Catch e m (Maybe a) -> Catch e (MaybeT m) a
liftListen :: Monad m => Listen w m (Maybe a) -> Listen w (MaybeT m) a
liftPass :: Monad m => Pass w m (Maybe a) -> Pass w (MaybeT m) a
mapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b
either :: (a -> c) -> (b -> c) -> Either a b -> c
lefts :: [Either a b] -> [a]
rights :: [Either a b] -> [b]
partitionEithers :: [Either a b] -> ([a], [b])
module Data.EitherR
maybe :: b -> (a -> b) -> Maybe a -> b
isJust :: Maybe a -> Bool
isNothing :: Maybe a -> Bool
fromMaybe :: a -> Maybe a -> a
maybeToList :: Maybe a -> [a]
listToMaybe :: [a] -> Maybe a
catMaybes :: [Maybe a] -> [a]
mapMaybe :: (a -> Maybe b) -> [a] -> [b]
foldr1May :: (a -> a -> a) -> [a] -> Maybe a
minimumMay :: Ord a => [a] -> Maybe a
maximumMay :: Ord a => [a] -> Maybe a
findJustDef :: a -> (a -> Bool) -> [a] -> a
minimumDef :: Ord a => a -> [a] -> a
maximumDef :: Ord a => a -> [a] -> a
foldr1Def :: a -> (a -> a -> a) -> [a] -> a
tailMay :: [a] -> Maybe [a]
tailDef :: [a] -> [a] -> [a]
tailSafe :: [a] -> [a]
initMay :: [a] -> Maybe [a]
initDef :: [a] -> [a] -> [a]
initSafe :: [a] -> [a]
headMay :: [a] -> Maybe a
lastMay :: [a] -> Maybe a
headDef :: a -> [a] -> a
lastDef :: a -> [a] -> a
foldl1May' :: (a -> a -> a) -> [a] -> Maybe a
fromJustDef :: a -> Maybe a -> a
atMay :: [a] -> Int -> Maybe a
atDef :: a -> [a] -> Int -> a
readMay :: Read a => String -> Maybe a
readDef :: Read a => a -> String -> a
lookupJustDef :: Eq a => b -> a -> [(a, b)] -> b
foldl1Def' :: a -> (a -> a -> a) -> [a] -> a

Re-exports

module Control.Error.Safe

module Control.Error.Script

module Control.Error.Util

newtype ExceptT e (m :: Type -> Type) a Source #

A monad transformer that adds exceptions to other monads.

ExceptT constructs a monad parameterized over two things:

e - The exception type.
m - The inner monad.

The return function yields a computation that produces the given value, while >>= sequences two subcomputations, exiting on the first exception.

Constructors

ExceptT (m (Either e a))

Instances

Instances details

MonadTrans (ExceptT e)
Instance details Defined in Control.Monad.Trans.Except Methods lift :: Monad m => m a -> ExceptT e m a Source #
MonadFail m => MonadFail (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods fail :: String -> ExceptT e m a Source #
MonadFix m => MonadFix (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods mfix :: (a -> ExceptT e m a) -> ExceptT e m a Source #
MonadIO m => MonadIO (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods liftIO :: IO a -> ExceptT e m a Source #
MonadZip m => MonadZip (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods mzip :: ExceptT e m a -> ExceptT e m b -> ExceptT e m (a, b) Source # mzipWith :: (a -> b -> c) -> ExceptT e m a -> ExceptT e m b -> ExceptT e m c Source # munzip :: ExceptT e m (a, b) -> (ExceptT e m a, ExceptT e m b) Source #
Foldable f => Foldable (ExceptT e f)
Instance details Defined in Control.Monad.Trans.Except Methods fold :: Monoid m => ExceptT e f m -> m Source # foldMap :: Monoid m => (a -> m) -> ExceptT e f a -> m Source # foldMap' :: Monoid m => (a -> m) -> ExceptT e f a -> m Source # foldr :: (a -> b -> b) -> b -> ExceptT e f a -> b Source # foldr' :: (a -> b -> b) -> b -> ExceptT e f a -> b Source # foldl :: (b -> a -> b) -> b -> ExceptT e f a -> b Source # foldl' :: (b -> a -> b) -> b -> ExceptT e f a -> b Source # foldr1 :: (a -> a -> a) -> ExceptT e f a -> a Source # foldl1 :: (a -> a -> a) -> ExceptT e f a -> a Source # toList :: ExceptT e f a -> [a] Source # null :: ExceptT e f a -> Bool Source # length :: ExceptT e f a -> Int Source # elem :: Eq a => a -> ExceptT e f a -> Bool Source # maximum :: Ord a => ExceptT e f a -> a Source # minimum :: Ord a => ExceptT e f a -> a Source # sum :: Num a => ExceptT e f a -> a Source # product :: Num a => ExceptT e f a -> a Source #
(Eq e, Eq1 m) => Eq1 (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods liftEq :: (a -> b -> Bool) -> ExceptT e m a -> ExceptT e m b -> Bool Source #
(Ord e, Ord1 m) => Ord1 (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods liftCompare :: (a -> b -> Ordering) -> ExceptT e m a -> ExceptT e m b -> Ordering Source #
(Read e, Read1 m) => Read1 (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (ExceptT e m a) Source # liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [ExceptT e m a] Source # liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (ExceptT e m a) Source # liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [ExceptT e m a] Source #
(Show e, Show1 m) => Show1 (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> ExceptT e m a -> ShowS Source # liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [ExceptT e m a] -> ShowS Source #
Contravariant m => Contravariant (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods contramap :: (a' -> a) -> ExceptT e m a -> ExceptT e m a' Source # (>$) :: b -> ExceptT e m b -> ExceptT e m a Source #
Traversable f => Traversable (ExceptT e f)
Instance details Defined in Control.Monad.Trans.Except Methods traverse :: Applicative f0 => (a -> f0 b) -> ExceptT e f a -> f0 (ExceptT e f b) Source # sequenceA :: Applicative f0 => ExceptT e f (f0 a) -> f0 (ExceptT e f a) Source # mapM :: Monad m => (a -> m b) -> ExceptT e f a -> m (ExceptT e f b) Source # sequence :: Monad m => ExceptT e f (m a) -> m (ExceptT e f a) Source #
(Functor m, Monad m, Monoid e) => Alternative (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods empty :: ExceptT e m a Source # (<\|>) :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a Source # some :: ExceptT e m a -> ExceptT e m [a] Source # many :: ExceptT e m a -> ExceptT e m [a] Source #
(Functor m, Monad m) => Applicative (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods pure :: a -> ExceptT e m a Source # (<>) :: ExceptT e m (a -> b) -> ExceptT e m a -> ExceptT e m b Source # liftA2 :: (a -> b -> c) -> ExceptT e m a -> ExceptT e m b -> ExceptT e m c Source # (>) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m b Source # (<*) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m a Source #
Functor m => Functor (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods fmap :: (a -> b) -> ExceptT e m a -> ExceptT e m b Source # (<$) :: a -> ExceptT e m b -> ExceptT e m a Source #
Monad m => Monad (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods (>>=) :: ExceptT e m a -> (a -> ExceptT e m b) -> ExceptT e m b Source # (>>) :: ExceptT e m a -> ExceptT e m b -> ExceptT e m b Source # return :: a -> ExceptT e m a Source #
(Monad m, Monoid e) => MonadPlus (ExceptT e m)
Instance details Defined in Control.Monad.Trans.Except Methods mzero :: ExceptT e m a Source # mplus :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a Source #
MonadCatch m => MonadCatch (ExceptT e m)	Catches exceptions from the base monad.
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e0 => ExceptT e m a -> (e0 -> ExceptT e m a) -> ExceptT e m a Source #
MonadMask m => MonadMask (ExceptT e m)	Since: exceptions-0.9.0
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. ExceptT e m a -> ExceptT e m a) -> ExceptT e m b) -> ExceptT e m b Source # uninterruptibleMask :: ((forall a. ExceptT e m a -> ExceptT e m a) -> ExceptT e m b) -> ExceptT e m b Source # generalBracket :: ExceptT e m a -> (a -> ExitCase b -> ExceptT e m c) -> (a -> ExceptT e m b) -> ExceptT e m (b, c) Source #
MonadThrow m => MonadThrow (ExceptT e m)	Throws exceptions into the base monad.
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e0 => e0 -> ExceptT e m a Source #
(Read e, Read1 m, Read a) => Read (ExceptT e m a)
Instance details Defined in Control.Monad.Trans.Except Methods readsPrec :: Int -> ReadS (ExceptT e m a) Source # readList :: ReadS [ExceptT e m a] Source # readPrec :: ReadPrec (ExceptT e m a) Source # readListPrec :: ReadPrec [ExceptT e m a] Source #
(Show e, Show1 m, Show a) => Show (ExceptT e m a)
Instance details Defined in Control.Monad.Trans.Except Methods showsPrec :: Int -> ExceptT e m a -> ShowS Source # show :: ExceptT e m a -> String Source # showList :: [ExceptT e m a] -> ShowS Source #
(Eq e, Eq1 m, Eq a) => Eq (ExceptT e m a)
Instance details Defined in Control.Monad.Trans.Except Methods (==) :: ExceptT e m a -> ExceptT e m a -> Bool Source # (/=) :: ExceptT e m a -> ExceptT e m a -> Bool Source #
(Ord e, Ord1 m, Ord a) => Ord (ExceptT e m a)
Instance details Defined in Control.Monad.Trans.Except Methods compare :: ExceptT e m a -> ExceptT e m a -> Ordering Source # (<) :: ExceptT e m a -> ExceptT e m a -> Bool Source # (<=) :: ExceptT e m a -> ExceptT e m a -> Bool Source # (>) :: ExceptT e m a -> ExceptT e m a -> Bool Source # (>=) :: ExceptT e m a -> ExceptT e m a -> Bool Source # max :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a Source # min :: ExceptT e m a -> ExceptT e m a -> ExceptT e m a Source #

throwE :: forall (m :: Type -> Type) e a. Monad m => e -> ExceptT e m a Source #

Signal an exception value e.

```
runExceptT (throwE e) = return (Left e)
```
```
throwE e >>= m = throwE e
```

runExceptT :: ExceptT e m a -> m (Either e a) Source #

The inverse of ExceptT.

mapExceptT :: (m (Either e a) -> n (Either e' b)) -> ExceptT e m a -> ExceptT e' n b Source #

Map the unwrapped computation using the given function.

runExceptT (mapExceptT f m) = f (runExceptT m)

withExceptT :: forall (m :: Type -> Type) e e' a. Functor m => (e -> e') -> ExceptT e m a -> ExceptT e' m a Source #

Transform any exceptions thrown by the computation using the given function.

catchE Source #

Arguments

:: forall (m :: Type -> Type) e a e'. Monad m
=> ExceptT e m a	the inner computation
-> (e -> ExceptT e' m a)	a handler for exceptions in the inner computation
-> ExceptT e' m a

Handle an exception.

```
catchE (lift m) h = lift m
```
```
catchE (throwE e) h = h e
```

newtype MaybeT (m :: Type -> Type) a Source #

The parameterizable maybe monad, obtained by composing an arbitrary monad with the Maybe monad.

Computations are actions that may produce a value or exit.

The return function yields a computation that produces that value, while >>= sequences two subcomputations, exiting if either computation does.

Constructors

MaybeT
Fields runMaybeT :: m (Maybe a)

Instances

Instances details

MonadTrans MaybeT
Instance details Defined in Control.Monad.Trans.Maybe Methods lift :: Monad m => m a -> MaybeT m a Source #
Monad m => MonadFail (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods fail :: String -> MaybeT m a Source #
MonadFix m => MonadFix (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods mfix :: (a -> MaybeT m a) -> MaybeT m a Source #
MonadIO m => MonadIO (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods liftIO :: IO a -> MaybeT m a Source #
MonadZip m => MonadZip (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods mzip :: MaybeT m a -> MaybeT m b -> MaybeT m (a, b) Source # mzipWith :: (a -> b -> c) -> MaybeT m a -> MaybeT m b -> MaybeT m c Source # munzip :: MaybeT m (a, b) -> (MaybeT m a, MaybeT m b) Source #
Foldable f => Foldable (MaybeT f)
Instance details Defined in Control.Monad.Trans.Maybe Methods fold :: Monoid m => MaybeT f m -> m Source # foldMap :: Monoid m => (a -> m) -> MaybeT f a -> m Source # foldMap' :: Monoid m => (a -> m) -> MaybeT f a -> m Source # foldr :: (a -> b -> b) -> b -> MaybeT f a -> b Source # foldr' :: (a -> b -> b) -> b -> MaybeT f a -> b Source # foldl :: (b -> a -> b) -> b -> MaybeT f a -> b Source # foldl' :: (b -> a -> b) -> b -> MaybeT f a -> b Source # foldr1 :: (a -> a -> a) -> MaybeT f a -> a Source # foldl1 :: (a -> a -> a) -> MaybeT f a -> a Source # toList :: MaybeT f a -> [a] Source # null :: MaybeT f a -> Bool Source # length :: MaybeT f a -> Int Source # elem :: Eq a => a -> MaybeT f a -> Bool Source # maximum :: Ord a => MaybeT f a -> a Source # minimum :: Ord a => MaybeT f a -> a Source # sum :: Num a => MaybeT f a -> a Source # product :: Num a => MaybeT f a -> a Source #
Eq1 m => Eq1 (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods liftEq :: (a -> b -> Bool) -> MaybeT m a -> MaybeT m b -> Bool Source #
Ord1 m => Ord1 (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods liftCompare :: (a -> b -> Ordering) -> MaybeT m a -> MaybeT m b -> Ordering Source #
Read1 m => Read1 (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS (MaybeT m a) Source # liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [MaybeT m a] Source # liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec (MaybeT m a) Source # liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [MaybeT m a] Source #
Show1 m => Show1 (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> MaybeT m a -> ShowS Source # liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [MaybeT m a] -> ShowS Source #
Contravariant m => Contravariant (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods contramap :: (a' -> a) -> MaybeT m a -> MaybeT m a' Source # (>$) :: b -> MaybeT m b -> MaybeT m a Source #
Traversable f => Traversable (MaybeT f)
Instance details Defined in Control.Monad.Trans.Maybe Methods traverse :: Applicative f0 => (a -> f0 b) -> MaybeT f a -> f0 (MaybeT f b) Source # sequenceA :: Applicative f0 => MaybeT f (f0 a) -> f0 (MaybeT f a) Source # mapM :: Monad m => (a -> m b) -> MaybeT f a -> m (MaybeT f b) Source # sequence :: Monad m => MaybeT f (m a) -> m (MaybeT f a) Source #
(Functor m, Monad m) => Alternative (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods empty :: MaybeT m a Source # (<\|>) :: MaybeT m a -> MaybeT m a -> MaybeT m a Source # some :: MaybeT m a -> MaybeT m [a] Source # many :: MaybeT m a -> MaybeT m [a] Source #
(Functor m, Monad m) => Applicative (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods pure :: a -> MaybeT m a Source # (<>) :: MaybeT m (a -> b) -> MaybeT m a -> MaybeT m b Source # liftA2 :: (a -> b -> c) -> MaybeT m a -> MaybeT m b -> MaybeT m c Source # (>) :: MaybeT m a -> MaybeT m b -> MaybeT m b Source # (<*) :: MaybeT m a -> MaybeT m b -> MaybeT m a Source #
Functor m => Functor (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods fmap :: (a -> b) -> MaybeT m a -> MaybeT m b Source # (<$) :: a -> MaybeT m b -> MaybeT m a Source #
Monad m => Monad (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods (>>=) :: MaybeT m a -> (a -> MaybeT m b) -> MaybeT m b Source # (>>) :: MaybeT m a -> MaybeT m b -> MaybeT m b Source # return :: a -> MaybeT m a Source #
Monad m => MonadPlus (MaybeT m)
Instance details Defined in Control.Monad.Trans.Maybe Methods mzero :: MaybeT m a Source # mplus :: MaybeT m a -> MaybeT m a -> MaybeT m a Source #
MonadCatch m => MonadCatch (MaybeT m)	Catches exceptions from the base monad.
Instance details Defined in Control.Monad.Catch Methods catch :: Exception e => MaybeT m a -> (e -> MaybeT m a) -> MaybeT m a Source #
MonadMask m => MonadMask (MaybeT m)	Since: exceptions-0.10.0
Instance details Defined in Control.Monad.Catch Methods mask :: ((forall a. MaybeT m a -> MaybeT m a) -> MaybeT m b) -> MaybeT m b Source # uninterruptibleMask :: ((forall a. MaybeT m a -> MaybeT m a) -> MaybeT m b) -> MaybeT m b Source # generalBracket :: MaybeT m a -> (a -> ExitCase b -> MaybeT m c) -> (a -> MaybeT m b) -> MaybeT m (b, c) Source #
MonadThrow m => MonadThrow (MaybeT m)	Throws exceptions into the base monad.
Instance details Defined in Control.Monad.Catch Methods throwM :: Exception e => e -> MaybeT m a Source #
(Read1 m, Read a) => Read (MaybeT m a)
Instance details Defined in Control.Monad.Trans.Maybe Methods readsPrec :: Int -> ReadS (MaybeT m a) Source # readList :: ReadS [MaybeT m a] Source # readPrec :: ReadPrec (MaybeT m a) Source # readListPrec :: ReadPrec [MaybeT m a] Source #
(Show1 m, Show a) => Show (MaybeT m a)
Instance details Defined in Control.Monad.Trans.Maybe Methods showsPrec :: Int -> MaybeT m a -> ShowS Source # show :: MaybeT m a -> String Source # showList :: [MaybeT m a] -> ShowS Source #
(Eq1 m, Eq a) => Eq (MaybeT m a)
Instance details Defined in Control.Monad.Trans.Maybe Methods (==) :: MaybeT m a -> MaybeT m a -> Bool Source # (/=) :: MaybeT m a -> MaybeT m a -> Bool Source #
(Ord1 m, Ord a) => Ord (MaybeT m a)
Instance details Defined in Control.Monad.Trans.Maybe Methods compare :: MaybeT m a -> MaybeT m a -> Ordering Source # (<) :: MaybeT m a -> MaybeT m a -> Bool Source # (<=) :: MaybeT m a -> MaybeT m a -> Bool Source # (>) :: MaybeT m a -> MaybeT m a -> Bool Source # (>=) :: MaybeT m a -> MaybeT m a -> Bool Source # max :: MaybeT m a -> MaybeT m a -> MaybeT m a Source # min :: MaybeT m a -> MaybeT m a -> MaybeT m a Source #

liftCallCC :: CallCC m (Maybe a) (Maybe b) -> CallCC (MaybeT m) a b Source #

Lift a callCC operation to the new monad.

liftCatch :: Catch e m (Maybe a) -> Catch e (MaybeT m) a Source #

Lift a catchE operation to the new monad.

liftListen :: Monad m => Listen w m (Maybe a) -> Listen w (MaybeT m) a Source #

Lift a listen operation to the new monad.

liftPass :: Monad m => Pass w m (Maybe a) -> Pass w (MaybeT m) a Source #

Lift a pass operation to the new monad.

mapMaybeT :: (m (Maybe a) -> n (Maybe b)) -> MaybeT m a -> MaybeT n b Source #

Transform the computation inside a MaybeT.

runMaybeT (mapMaybeT f m) = f (runMaybeT m)

either :: (a -> c) -> (b -> c) -> Either a b -> c Source #

Case analysis for the Either type. If the value is Left a, apply the first function to a; if it is Right b, apply the second function to b.

Examples

Expand

We create two values of type Either String Int, one using the Left constructor and another using the Right constructor. Then we apply "either" the length function (if we have a String) or the "times-two" function (if we have an Int):

>>> let s = Left "foo" :: Either String Int
>>> let n = Right 3 :: Either String Int
>>> either length (*2) s
3
>>> either length (*2) n
6

lefts :: [Either a b] -> [a] Source #

Extracts from a list of Either all the Left elements. All the Left elements are extracted in order.

Examples

Expand

Basic usage:

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]
>>> lefts list
["foo","bar","baz"]

rights :: [Either a b] -> [b] Source #

Extracts from a list of Either all the Right elements. All the Right elements are extracted in order.

Examples

Expand

Basic usage:

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]
>>> rights list
[3,7]

partitionEithers :: [Either a b] -> ([a], [b]) Source #

Partitions a list of Either into two lists. All the Left elements are extracted, in order, to the first component of the output. Similarly the Right elements are extracted to the second component of the output.

Examples

Expand

Basic usage:

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]
>>> partitionEithers list
(["foo","bar","baz"],[3,7])

The pair returned by partitionEithers x should be the same pair as (lefts x, rights x):

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]
>>> partitionEithers list == (lefts list, rights list)
True

module Data.EitherR

maybe :: b -> (a -> b) -> Maybe a -> b Source #

The maybe function takes a default value, a function, and a Maybe value. If the Maybe value is Nothing, the function returns the default value. Otherwise, it applies the function to the value inside the Just and returns the result.

Examples

Expand

Basic usage:

>>> maybe False odd (Just 3)
True

>>> maybe False odd Nothing
False

Read an integer from a string using readMaybe. If we succeed, return twice the integer; that is, apply (*2) to it. If instead we fail to parse an integer, return 0 by default:

>>> import Text.Read ( readMaybe )
>>> maybe 0 (*2) (readMaybe "5")
10
>>> maybe 0 (*2) (readMaybe "")
0

Apply show to a Maybe Int. If we have Just n, we want to show the underlying Int n. But if we have Nothing, we return the empty string instead of (for example) "Nothing":

>>> maybe "" show (Just 5)
"5"
>>> maybe "" show Nothing
""

isJust :: Maybe a -> Bool Source #

The isJust function returns True iff its argument is of the form Just _.

Examples

Expand

Basic usage:

>>> isJust (Just 3)
True

>>> isJust (Just ())
True

>>> isJust Nothing
False

Only the outer constructor is taken into consideration:

>>> isJust (Just Nothing)
True

isNothing :: Maybe a -> Bool Source #

The isNothing function returns True iff its argument is Nothing.

Examples

Expand

Basic usage:

>>> isNothing (Just 3)
False

>>> isNothing (Just ())
False

>>> isNothing Nothing
True

Only the outer constructor is taken into consideration:

>>> isNothing (Just Nothing)
False

fromMaybe :: a -> Maybe a -> a Source #

The fromMaybe function takes a default value and a Maybe value. If the Maybe is Nothing, it returns the default value; otherwise, it returns the value contained in the Maybe.

Examples

Expand

Basic usage:

>>> fromMaybe "" (Just "Hello, World!")
"Hello, World!"

>>> fromMaybe "" Nothing
""

Read an integer from a string using readMaybe. If we fail to parse an integer, we want to return 0 by default:

>>> import Text.Read ( readMaybe )
>>> fromMaybe 0 (readMaybe "5")
5
>>> fromMaybe 0 (readMaybe "")
0

maybeToList :: Maybe a -> [a] Source #

The maybeToList function returns an empty list when given Nothing or a singleton list when given Just.

Examples

Expand

Basic usage:

>>> maybeToList (Just 7)
[7]

>>> maybeToList Nothing
[]

One can use maybeToList to avoid pattern matching when combined with a function that (safely) works on lists:

>>> import Text.Read ( readMaybe )
>>> sum $ maybeToList (readMaybe "3")
3
>>> sum $ maybeToList (readMaybe "")
0

listToMaybe :: [a] -> Maybe a Source #

The listToMaybe function returns Nothing on an empty list or Just a where a is the first element of the list.

Examples

Expand

Basic usage:

>>> listToMaybe []
Nothing

>>> listToMaybe [9]
Just 9

>>> listToMaybe [1,2,3]
Just 1

Composing maybeToList with listToMaybe should be the identity on singleton/empty lists:

>>> maybeToList $ listToMaybe [5]
[5]
>>> maybeToList $ listToMaybe []
[]

But not on lists with more than one element:

>>> maybeToList $ listToMaybe [1,2,3]
[1]

catMaybes :: [Maybe a] -> [a] Source #

The catMaybes function takes a list of Maybes and returns a list of all the Just values.

Examples

Expand

Basic usage:

>>> catMaybes [Just 1, Nothing, Just 3]
[1,3]

When constructing a list of Maybe values, catMaybes can be used to return all of the "success" results (if the list is the result of a map, then mapMaybe would be more appropriate):

>>> import Text.Read ( readMaybe )
>>> [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]
[Just 1,Nothing,Just 3]
>>> catMaybes $ [readMaybe x :: Maybe Int | x <- ["1", "Foo", "3"] ]
[1,3]

mapMaybe :: (a -> Maybe b) -> [a] -> [b] Source #

The mapMaybe function is a version of map which can throw out elements. In particular, the functional argument returns something of type Maybe b. If this is Nothing, no element is added on to the result list. If it is Just b, then b is included in the result list.

Examples

Expand

Using mapMaybe f x is a shortcut for catMaybes $ map f x in most cases:

>>> import Text.Read ( readMaybe )
>>> let readMaybeInt = readMaybe :: String -> Maybe Int
>>> mapMaybe readMaybeInt ["1", "Foo", "3"]
[1,3]
>>> catMaybes $ map readMaybeInt ["1", "Foo", "3"]
[1,3]

If we map the Just constructor, the entire list should be returned:

>>> mapMaybe Just [1,2,3]
[1,2,3]

foldr1May :: (a -> a -> a) -> [a] -> Maybe a Source #

minimumMay :: Ord a => [a] -> Maybe a Source #

maximumMay :: Ord a => [a] -> Maybe a Source #

findJustDef :: a -> (a -> Bool) -> [a] -> a Source #

minimumDef :: Ord a => a -> [a] -> a Source #

New users are recommended to use minimumBound or maximumBound instead.

maximumDef :: Ord a => a -> [a] -> a Source #

New users are recommended to use minimumBound or maximumBound instead.

foldr1Def :: a -> (a -> a -> a) -> [a] -> a Source #

tailMay :: [a] -> Maybe [a] Source #

tailMay [] = Nothing
tailMay [1,3,4] = Just [3,4]

tailDef :: [a] -> [a] -> [a] Source #

tailDef [12] [] = [12]
tailDef [12] [1,3,4] = [3,4]

tailSafe :: [a] -> [a] Source #

tailSafe [] = []
tailSafe [1,3,4] = [3,4]

initMay :: [a] -> Maybe [a] Source #

initDef :: [a] -> [a] -> [a] Source #

initSafe :: [a] -> [a] Source #

headMay :: [a] -> Maybe a Source #

lastMay :: [a] -> Maybe a Source #

headDef :: a -> [a] -> a Source #

lastDef :: a -> [a] -> a Source #

foldl1May' :: (a -> a -> a) -> [a] -> Maybe a Source #

fromJustDef :: a -> Maybe a -> a Source #

An alternative name for fromMaybe, to fit the naming scheme of this package. Generally using fromMaybe directly would be considered better style.

atMay :: [a] -> Int -> Maybe a Source #

atDef :: a -> [a] -> Int -> a Source #

readMay :: Read a => String -> Maybe a Source #

readDef :: Read a => a -> String -> a Source #

lookupJustDef :: Eq a => b -> a -> [(a, b)] -> b Source #

foldl1Def' :: a -> (a -> a -> a) -> [a] -> a Source #