Haskell Core Libraries (base package)ContentsIndex
Prelude
Portability portable
Stability provisional
Maintainer libraries@haskell.org
Contents
Basic data types
Basic type classes
List operations
Converting to and from String
Simple I/O operations
Monads
Miscellaneous functions
Description
The Prelude: a standard module imported by default into all Haskell modules. For more documentation, see the Haskell 98 Report http://www.haskell.org/onlinereport/.
Synopsis
data Bool
= False
| True
data Maybe a
= Nothing
| Just a
data Either a b
= Left a
| Right b
data Ordering
= LT
| EQ
| GT
data Char
type String = [Char]
data Int
data Integer
data Float
data Double
data IO a
type Rational = Ratio Integer
module Data.Tuple
class Eq a where
(==) :: a -> a -> Bool
(/=) :: a -> a -> Bool
class (Eq a) => Ord a where
max :: a -> a -> a
min :: a -> a -> a
(<) :: a -> a -> Bool
(<=) :: a -> a -> Bool
(>) :: a -> a -> Bool
(>=) :: a -> a -> Bool
compare :: a -> a -> Ordering
class Enum a where
enumFromThenTo :: a -> a -> a -> [a]
enumFromTo :: a -> a -> [a]
enumFromThen :: a -> a -> [a]
enumFrom :: a -> [a]
fromEnum :: a -> Int
toEnum :: Int -> a
succ :: a -> a
pred :: a -> a
class Bounded a where
minBound :: a
maxBound :: a
class (Eq a, Show a) => Num a where
fromInteger :: Integer -> a
abs :: a -> a
signum :: a -> a
negate :: a -> a
(+) :: a -> a -> a
(-) :: a -> a -> a
(*) :: a -> a -> a
class (Num a, Ord a) => Real a where
toRational :: a -> Rational
class (Real a, Enum a) => Integral a where
toInteger :: a -> Integer
quotRem :: a -> a -> (a, a)
divMod :: a -> a -> (a, a)
quot :: a -> a -> a
rem :: a -> a -> a
div :: a -> a -> a
mod :: a -> a -> a
class (Num a) => Fractional a where
fromRational :: Rational -> a
recip :: a -> a
(/) :: a -> a -> a
class (Fractional a) => Floating a where
asinh :: a -> a
acosh :: a -> a
atanh :: a -> a
sinh :: a -> a
cosh :: a -> a
tanh :: a -> a
asin :: a -> a
acos :: a -> a
atan :: a -> a
sin :: a -> a
cos :: a -> a
tan :: a -> a
(**) :: a -> a -> a
logBase :: a -> a -> a
exp :: a -> a
log :: a -> a
sqrt :: a -> a
pi :: a
class (Real a, Fractional a) => RealFrac a where
ceiling :: (Integral b) => a -> b
floor :: (Integral b) => a -> b
truncate :: (Integral b) => a -> b
round :: (Integral b) => a -> b
properFraction :: (Integral b) => a -> (b, a)
class (RealFrac a, Floating a) => RealFloat a where
atan2 :: a -> a -> a
isNaN :: a -> Bool
isInfinite :: a -> Bool
isDenormalized :: a -> Bool
isNegativeZero :: a -> Bool
isIEEE :: a -> Bool
scaleFloat :: Int -> a -> a
significand :: a -> a
exponent :: a -> Int
encodeFloat :: Integer -> Int -> a
decodeFloat :: a -> (Integer, Int)
floatRange :: a -> (Int, Int)
floatDigits :: a -> Int
floatRadix :: a -> Integer
map :: (a -> b) -> [a] -> [b]
(++) :: [a] -> [a] -> [a]
filter :: (a -> Bool) -> [a] -> [a]
concat :: [[a]] -> [a]
head :: [a] -> a
last :: [a] -> a
tail :: [a] -> [a]
init :: [a] -> [a]
null :: [a] -> Bool
length :: [a] -> Int
(!!) :: [a] -> Int -> a
foldl :: (a -> b -> a) -> a -> [b] -> a
foldl1 :: (a -> a -> a) -> [a] -> a
scanl :: (a -> b -> a) -> a -> [b] -> [a]
scanl1 :: (a -> a -> a) -> [a] -> [a]
foldr :: (a -> b -> b) -> b -> [a] -> b
foldr1 :: (a -> a -> a) -> [a] -> a
scanr :: (a -> b -> b) -> b -> [a] -> [b]
scanr1 :: (a -> a -> a) -> [a] -> [a]
iterate :: (a -> a) -> a -> [a]
repeat :: a -> [a]
replicate :: Int -> a -> [a]
cycle :: [a] -> [a]
take :: Int -> [b] -> [b]
drop :: Int -> [b] -> [b]
splitAt :: Int -> [b] -> ([b], [b])
takeWhile :: (a -> Bool) -> [a] -> [a]
dropWhile :: (a -> Bool) -> [a] -> [a]
span :: (a -> Bool) -> [a] -> ([a], [a])
break :: (a -> Bool) -> [a] -> ([a], [a])
reverse :: [a] -> [a]
and :: [Bool] -> Bool
or :: [Bool] -> Bool
any :: (a -> Bool) -> [a] -> Bool
all :: (a -> Bool) -> [a] -> Bool
elem :: (Eq a) => a -> [a] -> Bool
notElem :: (Eq a) => a -> [a] -> Bool
lookup :: (Eq a) => a -> [(a, b)] -> Maybe b
maximum :: (Ord a) => [a] -> a
minimum :: (Ord a) => [a] -> a
concatMap :: (a -> [b]) -> [a] -> [b]
zip :: [a] -> [b] -> [(a, b)]
zip3 :: [a] -> [b] -> [c] -> [(a, b, c)]
zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
zipWith3 :: (a -> b -> c -> d) -> [a] -> [b] -> [c] -> [d]
unzip :: [(a, b)] -> ([a], [b])
unzip3 :: [(a, b, c)] -> ([a], [b], [c])
lines :: String -> [String]
words :: String -> [String]
unlines :: [String] -> String
unwords :: [String] -> String
sum :: (Num a) => [a] -> a
product :: (Num a) => [a] -> a
type ReadS a = String -> [(a, String)]
type ShowS = String -> String
class Read a where
readList :: ReadS [a]
readsPrec :: Int -> ReadS a
class Show a where
showList :: [a] -> ShowS
show :: a -> String
showsPrec :: Int -> a -> ShowS
reads :: (Read a) => ReadS a
shows :: (Show a) => a -> ShowS
read :: (Read a) => String -> a
lex :: ReadS String
showChar :: Char -> ShowS
showString :: String -> ShowS
readParen :: Bool -> ReadS a -> ReadS a
showParen :: Bool -> ShowS -> ShowS
ioError :: Exception -> IO a
userError :: String -> IOError
catch :: IO a -> (Exception -> IO a) -> IO a
type FilePath = String
type IOError = Exception
putChar :: Char -> IO ()
putStr :: String -> IO ()
putStrLn :: String -> IO ()
print :: (Show a) => a -> IO ()
getChar :: IO Char
getLine :: IO String
getContents :: IO String
interact :: (String -> String) -> IO ()
readFile :: FilePath -> IO String
writeFile :: FilePath -> String -> IO ()
appendFile :: FilePath -> String -> IO ()
readIO :: (Read a) => String -> IO a
readLn :: (Read a) => IO a
class Monad m where
fail :: String -> m a
return :: a -> m a
(>>) :: m a -> m b -> m b
(>>=) :: m a -> (a -> m b) -> m b
class Functor f where
fmap :: (a -> b) -> f a -> f b
mapM :: (Monad m) => (a -> m b) -> [a] -> m [b]
mapM_ :: (Monad m) => (a -> m b) -> [a] -> m ()
sequence :: (Monad m) => [m a] -> m [a]
sequence_ :: (Monad m) => [m a] -> m ()
(=<<) :: (Monad m) => (a -> m b) -> m a -> m b
maybe :: b -> (a -> b) -> Maybe a -> b
either :: (a -> c) -> (b -> c) -> Either a b -> c
(&&) :: Bool -> Bool -> Bool
(||) :: Bool -> Bool -> Bool
not :: Bool -> Bool
otherwise :: Bool
subtract :: (Num a) => a -> a -> a
even :: (Integral a) => a -> Bool
odd :: (Integral a) => a -> Bool
gcd :: (Integral a) => a -> a -> a
lcm :: (Integral a) => a -> a -> a
(^) :: (Num a, Integral b) => a -> b -> a
(^^) :: (Fractional a, Integral b) => a -> b -> a
fromIntegral :: (Integral a, Num b) => a -> b
realToFrac :: (Real a, Fractional b) => a -> b
id :: a -> a
const :: a -> b -> a
(.) :: (b -> c) -> (a -> b) -> a -> c
flip :: (a -> b -> c) -> b -> a -> c
($) :: (a -> b) -> a -> b
until :: (a -> Bool) -> (a -> a) -> a -> a
asTypeOf :: a -> a -> a
error :: String -> a
undefined :: a
($!) :: (a -> b) -> a -> b
Basic data types
data Bool
The Bool type is an enumeration. It is defined with False first so that the corresponding Enum instance will give fromEnum False the value zero, and fromEnum True the value 1.
Constructors
False
True
Instances
IArray UArray Bool
(Ix ix) => Eq (UArray ix Bool)
(Ix ix) => Ord (UArray ix Bool)
(Ix ix, Show ix) => Show (UArray ix Bool)
MArray (STUArray s) Bool (ST s)
MArray IOUArray Bool IO
Typeable Bool
Arbitrary Bool
Testable Bool
Ix Bool
Bounded Bool
Enum Bool
Read Bool
Show Bool
Storable Bool
Random Bool
data Maybe a

The Maybe type encapsulates an optional value. A value of type Maybe a either contains a value of type a (represented as Just a), or it is empty (represented as Nothing). Using Maybe is a good way to deal with errors or exceptional cases without resorting to drastic measures such as error.

The Maybe type is also a monad. It is a simple kind of error monad, where all errors are represented by Nothing. A richer error monad can be built using the Either type.

Constructors
Nothing
Just a
Instances
MonadPlus Maybe
MonadFix Maybe
(Typeable a) => Typeable (Maybe a)
Functor Maybe
Monad Maybe
(Read a) => Read (Maybe a)
(Show a) => Show (Maybe a)
data Either a b

The Either type represents values with two possibilities: a value of type Either a b is either Left a or Right b.

The Either type is sometimes used to represent a value which is either correct or an error; by convention, the Left constructor is used to hold an error value and the Right constructor is used to hold a correct value (mnemonic: "right" also means "correct").

Constructors
Left a
Right b
Instances
Functor (Either e)
(Error e) => Monad (Either e)
(Error e) => MonadPlus (Either e)
(Error e) => MonadFix (Either e)
(Error e) => MonadError e (Either e)
(Typeable a, Typeable b) => Typeable (Either a b)
(Read a, Read b) => Read (Either a b)
(Show a, Show b) => Show (Either a b)
data Ordering
Represents an ordering relationship between two values: less than, equal to, or greater than. An Ordering is returned by compare.
Constructors
LT
EQ
GT
Instances
Typeable Ordering
Ix Ordering
Bounded Ordering
Enum Ordering
Read Ordering
Show Ordering
data Char

The character type Char is an enumeration whose values represent Unicode characters. A character literal in Haskell has type Char.

To convert a Char to or from an Int, use toEnum and fromEnum from the Enum class respectively (equivalently ord and chr also do the trick).

Instances
Error [Char]
IArray UArray Char
(Ix ix) => Eq (UArray ix Char)
(Ix ix) => Ord (UArray ix Char)
(Ix ix, Show ix) => Show (UArray ix Char)
MArray (STUArray s) Char (ST s)
(Ix ix, Show ix) => Show (DiffUArray ix Char)
IArray (IOToDiffArray IOUArray) Char
MArray IOUArray Char IO
Typeable Char
Ix Char
Eq Char
Ord Char
CCallable Char
CReturnable Char
Bounded Char
Enum Char
Read Char
Show Char
Storable Char
Random Char
HTML Char
type String = [Char]
A String is a list of characters. String constants in Haskell are values of type String.
data Int
A fixed-precision integer type with at least the range [-2^29 .. 2^29-1]. The exact range for a given implementation can be determined by using minBound and maxBound from the Bounded class.
Instances
IArray UArray Int
(Ix ix) => Eq (UArray ix Int)
(Ix ix) => Ord (UArray ix Int)
(Ix ix, Show ix) => Show (UArray ix Int)
MArray (STUArray s) Int (ST s)
(Ix ix, Show ix) => Show (DiffUArray ix Int)
IArray (IOToDiffArray IOUArray) Int
MArray IOUArray Int IO
Bits Int
Typeable Int
Arbitrary Int
Ix Int
Eq Int
Ord Int
CCallable Int
CReturnable Int
Bounded Int
Enum Int
Num Int
Read Int
Real Int
Integral Int
Show Int
Storable Int
Random Int
data Integer
Arbitrary-precision integers.
Instances
Bits Integer
Typeable Integer
Arbitrary Integer
Ix Integer
Eq Integer
Ord Integer
Num Integer
Enum Integer
Show Integer
Read Integer
Real Integer
Integral Integer
Random Integer
data Float
Single-precision floating point numbers.
Instances
IArray UArray Float
(Ix ix) => Eq (UArray ix Float)
(Ix ix) => Ord (UArray ix Float)
(Ix ix, Show ix) => Show (UArray ix Float)
MArray (STUArray s) Float (ST s)
(Ix ix, Show ix) => Show (DiffUArray ix Float)
IArray (IOToDiffArray IOUArray) Float
MArray IOUArray Float IO
Typeable Float
Arbitrary Float
CCallable Float
CReturnable Float
Eq Float
Ord Float
Num Float
Real Float
Fractional Float
RealFrac Float
Floating Float
RealFloat Float
Show Float
Enum Float
Read Float
Storable Float
Random Float
data Double
Double-precision floating point numbers.
Instances
IArray UArray Double
(Ix ix) => Eq (UArray ix Double)
(Ix ix) => Ord (UArray ix Double)
(Ix ix, Show ix) => Show (UArray ix Double)
MArray (STUArray s) Double (ST s)
(Ix ix, Show ix) => Show (DiffUArray ix Double)
IArray (IOToDiffArray IOUArray) Double
MArray IOUArray Double IO
Typeable Double
Arbitrary Double
CCallable Double
CReturnable Double
Eq Double
Ord Double
Num Double
Real Double
Fractional Double
Floating Double
RealFrac Double
RealFloat Double
Show Double
Enum Double
Read Double
Storable Double
Random Double
data IO a

A value of type IO a is a computation which, when performed, does some I/O before returning a value of type a.

There is really only one way to "perform" an I/O action: bind it to Main.main in your program. When your program is run, the I/O will be performed. It isn't possible to perform I/O from an arbitrary function, unless that function is itself in the IO monad and called at some point, directly or indirectly, from Main.main.

IO is a monad, so IO actions can be combined using either the do-notation or the >> and >>= operations from the Monad class.

Instances
MonadPlus IO
MonadError IOError IO
MonadFix IO
MonadIO IO
MArray IOArray e IO
MArray IOUArray Bool IO
MArray IOUArray Char IO
MArray IOUArray Int IO
MArray IOUArray Word IO
MArray IOUArray (Ptr a) IO
MArray IOUArray (FunPtr a) IO
MArray IOUArray Float IO
MArray IOUArray Double IO
MArray IOUArray (StablePtr a) IO
MArray IOUArray Int8 IO
MArray IOUArray Int16 IO
MArray IOUArray Int32 IO
MArray IOUArray Int64 IO
MArray IOUArray Word8 IO
MArray IOUArray Word16 IO
MArray IOUArray Word32 IO
MArray IOUArray Word64 IO
(Storable e) => MArray StorableArray e IO
(Typeable a) => Typeable (IO a)
Functor IO
Monad IO
type Rational = Ratio Integer
Arbitrary-precision rational numbers, represented as a ratio of two Integer values. A rational number may be constructed using the % operator.
module Data.Tuple
Basic type classes
class Eq a where

The Eq class defines equality (==) and inequality (/=). All the basic datatypes exported by the Prelude are instances of Eq, and Eq may be derived for any datatype whose constituents are also instances of Eq.

Minimal complete definition: either == or /=.

Methods
(==) :: a -> a -> Bool
(/=) :: a -> a -> Bool
Instances
Eq ThreadId
(Ix ix) => Eq (UArray ix Bool)
(Ix ix) => Eq (UArray ix Char)
(Ix ix) => Eq (UArray ix Int)
(Ix ix) => Eq (UArray ix Word)
(Ix ix) => Eq (UArray ix (Ptr a))
(Ix ix) => Eq (UArray ix (FunPtr a))
(Ix ix) => Eq (UArray ix Float)
(Ix ix) => Eq (UArray ix Double)
(Ix ix) => Eq (UArray ix (StablePtr a))
(Ix ix) => Eq (UArray ix Int8)
(Ix ix) => Eq (UArray ix Int16)
(Ix ix) => Eq (UArray ix Int32)
(Ix ix) => Eq (UArray ix Int64)
(Ix ix) => Eq (UArray ix Word8)
(Ix ix) => Eq (UArray ix Word16)
(Ix ix) => Eq (UArray ix Word32)
(Ix ix) => Eq (UArray ix Word64)
Eq TyCon
(Eq key, Eq elt) => Eq (FiniteMap key elt)
Eq PackedString
(Eq a) => Eq (Set a)
(Eq a) => Eq (Poly a)
Eq Errno
Eq (ForeignPtr a)
Eq (STArray s i e)
(Ix i, Eq e) => Eq (Array i e)
(Eq a) => Eq [a]
Eq ()
Eq Char
Eq Int
Eq Float
Eq Double
Eq HandlePosn
Eq (MVar a)
Eq Handle
Eq Exception
Eq IOException
Eq IOErrorType
Eq Int64
Eq Integer
Eq (STRef s a)
Eq (StablePtr a)
Eq Word64
Eq (StableName a)
class (Eq a) => Ord a where
Methods
max :: a -> a -> a
min :: a -> a -> a
(<) :: a -> a -> Bool
(<=) :: a -> a -> Bool
(>) :: a -> a -> Bool
(>=) :: a -> a -> Bool
compare :: a -> a -> Ordering
Instances
Ord ThreadId
(Ix ix) => Ord (UArray ix Bool)
(Ix ix) => Ord (UArray ix Char)
(Ix ix) => Ord (UArray ix Int)
(Ix ix) => Ord (UArray ix Word)
(Ix ix) => Ord (UArray ix (Ptr a))
(Ix ix) => Ord (UArray ix (FunPtr a))
(Ix ix) => Ord (UArray ix Float)
(Ix ix) => Ord (UArray ix Double)
(Ix ix) => Ord (UArray ix Int8)
(Ix ix) => Ord (UArray ix Int16)
(Ix ix) => Ord (UArray ix Int32)
(Ix ix) => Ord (UArray ix Int64)
(Ix ix) => Ord (UArray ix Word8)
(Ix ix) => Ord (UArray ix Word16)
(Ix ix) => Ord (UArray ix Word32)
(Ix ix) => Ord (UArray ix Word64)
Ord PackedString
(Ord a) => Ord (Poly a)
(Ix i, Ord e) =&g