Safe Haskell	Safe
Language	Haskell2010

Relude.Bool.Reexport

Description

This module reexports functions to work with Bool type.

Synopsis

guard :: Alternative f => Bool -> f ()
unless :: Applicative f => Bool -> f () -> f ()
when :: Applicative f => Bool -> f () -> f ()
otherwise :: Bool
data Bool
- = False
- | True
bool :: a -> a -> Bool -> a
(&&) :: Bool -> Bool -> Bool
(||) :: Bool -> Bool -> Bool
not :: Bool -> Bool

Documentation

guard :: Alternative f => Bool -> f () #

Conditional failure of Alternative computations. Defined by

guard True  = pure ()
guard False = empty

Examples

Expand

Common uses of guard include conditionally signaling an error in an error monad and conditionally rejecting the current choice in an Alternative-based parser.

As an example of signaling an error in the error monad Maybe, consider a safe division function safeDiv x y that returns Nothing when the denominator y is zero and Just (x `div` y) otherwise. For example:

>>> safeDiv 4 0
Nothing
>>> safeDiv 4 2
Just 2

A definition of safeDiv using guards, but not guard:

safeDiv :: Int -> Int -> Maybe Int
safeDiv x y | y /= 0    = Just (x `div` y)
            | otherwise = Nothing

A definition of safeDiv using guard and Monad do-notation:

safeDiv :: Int -> Int -> Maybe Int
safeDiv x y = do
  guard (y /= 0)
  return (x `div` y)

unless :: Applicative f => Bool -> f () -> f () #

The reverse of when.

when :: Applicative f => Bool -> f () -> f () #

Conditional execution of Applicative expressions. For example,

when debug (putStrLn "Debugging")

will output the string Debugging if the Boolean value debug is True, and otherwise do nothing.

otherwise :: Bool #

otherwise is defined as the value True. It helps to make guards more readable. eg.

 f x | x < 0     = ...
     | otherwise = ...

data Bool #

Constructors

False
True

Instances

Bounded Bool	Since: base-2.1
Instance details Defined in GHC.Enum Methods minBound :: Bool # maxBound :: Bool #
Enum Bool	Since: base-2.1
Instance details Defined in GHC.Enum Methods succ :: Bool -> Bool # pred :: Bool -> Bool # toEnum :: Int -> Bool # fromEnum :: Bool -> Int # enumFrom :: Bool -> [Bool] # enumFromThen :: Bool -> Bool -> [Bool] # enumFromTo :: Bool -> Bool -> [Bool] # enumFromThenTo :: Bool -> Bool -> Bool -> [Bool] #
Eq Bool
Instance details Defined in GHC.Classes Methods (==) :: Bool -> Bool -> Bool # (/=) :: Bool -> Bool -> Bool #
Data Bool	Since: base-4.0.0.0
Instance details Defined in Data.Data Methods gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> Bool -> c Bool # gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c Bool # toConstr :: Bool -> Constr # dataTypeOf :: Bool -> DataType # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c Bool) # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c Bool) # gmapT :: (forall b. Data b => b -> b) -> Bool -> Bool # gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Bool -> r # gmapQr :: (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Bool -> r # gmapQ :: (forall d. Data d => d -> u) -> Bool -> [u] # gmapQi :: Int -> (forall d. Data d => d -> u) -> Bool -> u # gmapM :: Monad m => (forall d. Data d => d -> m d) -> Bool -> m Bool # gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Bool -> m Bool # gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Bool -> m Bool #
Ord Bool
Instance details Defined in GHC.Classes Methods compare :: Bool -> Bool -> Ordering # (<) :: Bool -> Bool -> Bool # (<=) :: Bool -> Bool -> Bool # (>) :: Bool -> Bool -> Bool # (>=) :: Bool -> Bool -> Bool # max :: Bool -> Bool -> Bool # min :: Bool -> Bool -> Bool #
Read Bool	Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS Bool # readList :: ReadS [Bool] # readPrec :: ReadPrec Bool # readListPrec :: ReadPrec [Bool] #
Show Bool
Instance details Defined in GHC.Show Methods showsPrec :: Int -> Bool -> ShowS # show :: Bool -> String # showList :: [Bool] -> ShowS #
Ix Bool	Since: base-2.1
Instance details Defined in GHC.Arr Methods range :: (Bool, Bool) -> [Bool] # index :: (Bool, Bool) -> Bool -> Int # unsafeIndex :: (Bool, Bool) -> Bool -> Int inRange :: (Bool, Bool) -> Bool -> Bool # rangeSize :: (Bool, Bool) -> Int # unsafeRangeSize :: (Bool, Bool) -> Int
Generic Bool
Instance details Defined in GHC.Generics Associated Types type Rep Bool :: * -> * # Methods from :: Bool -> Rep Bool x # to :: Rep Bool x -> Bool #
SingKind Bool	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Associated Types type DemoteRep Bool :: * Methods fromSing :: Sing a -> DemoteRep Bool
Bits Bool	Interpret `Bool` as 1-bit bit-field Since: base-4.7.0.0
Instance details Defined in Data.Bits Methods (.&.) :: Bool -> Bool -> Bool # (.\|.) :: Bool -> Bool -> Bool # xor :: Bool -> Bool -> Bool # complement :: Bool -> Bool # shift :: Bool -> Int -> Bool # rotate :: Bool -> Int -> Bool # zeroBits :: Bool # bit :: Int -> Bool # setBit :: Bool -> Int -> Bool # clearBit :: Bool -> Int -> Bool # complementBit :: Bool -> Int -> Bool # testBit :: Bool -> Int -> Bool # bitSizeMaybe :: Bool -> Maybe Int # bitSize :: Bool -> Int # isSigned :: Bool -> Bool # shiftL :: Bool -> Int -> Bool # unsafeShiftL :: Bool -> Int -> Bool # shiftR :: Bool -> Int -> Bool # unsafeShiftR :: Bool -> Int -> Bool # rotateL :: Bool -> Int -> Bool # rotateR :: Bool -> Int -> Bool # popCount :: Bool -> Int #
FiniteBits Bool	Since: base-4.7.0.0
Instance details Defined in Data.Bits Methods finiteBitSize :: Bool -> Int # countLeadingZeros :: Bool -> Int # countTrailingZeros :: Bool -> Int #
NFData Bool
Instance details Defined in Control.DeepSeq Methods rnf :: Bool -> () #
Hashable Bool
Instance details Defined in Data.Hashable.Class Methods hashWithSalt :: Int -> Bool -> Int # hash :: Bool -> Int #
SingI False	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods sing :: Sing False
SingI True	Since: base-4.9.0.0
Instance details Defined in GHC.Generics Methods sing :: Sing True
type Rep Bool
Instance details Defined in GHC.Generics type Rep Bool = D1 (MetaData "Bool" "GHC.Types" "ghc-prim" False) (C1 (MetaCons "False" PrefixI False) (U1 :: * -> ) :+: C1 (MetaCons "True" PrefixI False) (U1 :: -> *))
data Sing (a :: Bool)
Instance details Defined in GHC.Generics data Sing (a :: Bool) where STrue :: Sing True SFalse :: Sing False
type DemoteRep Bool
Instance details Defined in GHC.Generics type DemoteRep Bool = Bool

bool :: a -> a -> Bool -> a #

Case analysis for the Bool type. bool x y p evaluates to x when p is False, and evaluates to y when p is True.

This is equivalent to if p then y else x; that is, one can think of it as an if-then-else construct with its arguments reordered.

Examples

Expand

Basic usage:

>>> bool "foo" "bar" True
"bar"
>>> bool "foo" "bar" False
"foo"

Confirm that bool x y p and if p then y else x are equivalent:

>>> let p = True; x = "bar"; y = "foo"
>>> bool x y p == if p then y else x
True
>>> let p = False
>>> bool x y p == if p then y else x
True

Since: base-4.7.0.0

(&&) :: Bool -> Bool -> Bool infixr 3 #

Boolean "and"

(||) :: Bool -> Bool -> Bool infixr 2 #

Boolean "or"

not :: Bool -> Bool #

Boolean "not"