Haskell Hierarchical Libraries (base package)ContentsIndex
Prelude
Portability portable
Stability stable
Maintainer libraries@haskell.org
Contents
Standard types, classes and related functions
Basic data types
Tuples
Basic type classes
Numbers
Numeric types
Numeric type classes
Numeric functions
Monads and functors
Miscellaneous functions
List operations
Reducing lists (folds)
Special folds
Building lists
Scans
Infinite lists
Sublists
Searching lists
Zipping and unzipping lists
Functions on strings
Converting to and from String
Basic Input and output
Simple I/O operations
Output functions
Input functions
Files
Exception handling in the I/O monad
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
(&&) :: Bool -> Bool -> Bool
(||) :: Bool -> Bool -> Bool
not :: Bool -> Bool
otherwise :: Bool
data Maybe a
= Nothing
| Just a
maybe :: b -> (a -> b) -> Maybe a -> b
data Either a b
= Left a
| Right b
either :: (a -> c) -> (b -> c) -> Either a b -> c
data Ordering
= LT
| EQ
| GT
data Char
type String = [Char]
fst :: (a, b) -> a
snd :: (a, b) -> b
curry :: ((a, b) -> c) -> a -> b -> c
uncurry :: (a -> b -> c) -> (a, b) -> c
class Eq a where
(==) :: a -> a -> Bool
(/=) :: a -> a -> Bool
class Eq a => Ord a where
compare :: a -> a -> Ordering
(<) :: a -> a -> Bool
(<=) :: a -> a -> Bool
(>) :: a -> a -> Bool
(>=) :: a -> a -> Bool
max :: a -> a -> a
min :: a -> a -> a
class Enum a where
succ :: a -> a
pred :: a -> a
toEnum :: Int -> a
fromEnum :: a -> Int
enumFrom :: a -> [a]
enumFromThen :: a -> a -> [a]
enumFromTo :: a -> a -> [a]
enumFromThenTo :: a -> a -> a -> [a]
class Bounded a where
minBound :: a
maxBound :: a
data Int
data Integer
data Float
data Double
type Rational = Ratio Integer
class (Eq a, Show a) => Num a where
(+) :: a -> a -> a
(-) :: a -> a -> a
(*) :: a -> a -> a
negate :: a -> a
abs :: a -> a
signum :: a -> a
fromInteger :: Integer -> a
class (Num a, Ord a) => Real a where
toRational :: a -> Rational
class (Real a, Enum a) => Integral a where
quot :: a -> a -> a
rem :: a -> a -> a
div :: a -> a -> a
mod :: a -> a -> a
quotRem :: a -> a -> (a, a)
divMod :: a -> a -> (a, a)
toInteger :: a -> Integer
class Num a => Fractional a where
(/) :: a -> a -> a
recip :: a -> a
fromRational :: Rational -> a
class Fractional a => Floating a where
pi :: a
exp :: a -> a
log :: a -> a
sqrt :: a -> a
(**) :: a -> a -> a
logBase :: a -> a -> a
sin :: a -> a
cos :: a -> a
tan :: a -> a
asin :: a -> a
acos :: a -> a
atan :: a -> a
sinh :: a -> a
cosh :: a -> a
tanh :: a -> a
asinh :: a -> a
acosh :: a -> a
atanh :: a -> a
class (Real a, Fractional a) => RealFrac a where
properFraction :: Integral b => a -> (b, a)
truncate :: Integral b => a -> b
round :: Integral b => a -> b
ceiling :: Integral b => a -> b
floor :: Integral b => a -> b
class (RealFrac a, Floating a) => RealFloat a where
floatRadix :: a -> Integer
floatDigits :: a -> Int
floatRange :: a -> (Int, Int)
decodeFloat :: a -> (Integer, Int)
encodeFloat :: Integer -> Int -> a
exponent :: a -> Int
significand :: a -> a
scaleFloat :: Int -> a -> a
isNaN :: a -> Bool
isInfinite :: a -> Bool
isDenormalized :: a -> Bool
isNegativeZero :: a -> Bool
isIEEE :: a -> Bool
atan2 :: a -> a -> a
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
class Monad m where
(>>=) :: m a -> (a -> m b) -> m b
(>>) :: m a -> m b -> m b
return :: a -> m a
fail :: String -> m a
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
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
map :: (a -> b) -> [a] -> [b]
(++) :: [a] -> [a] -> [a]
filter :: (a -> Bool) -> [a] -> [a]
head :: [a] -> a
last :: [a] -> a
tail :: [a] -> [a]
init :: [a] -> [a]
null :: [a] -> Bool
length :: [a] -> Int
(!!) :: [a] -> Int -> a
reverse :: [a] -> [a]
foldl :: (a -> b -> a) -> a -> [b] -> a
foldl1 :: (a -> a -> a) -> [a] -> a
foldr :: (a -> b -> b) -> b -> [a] -> b
foldr1 :: (a -> a -> a) -> [a] -> a
and :: [Bool] -> Bool
or :: [Bool] -> Bool
any :: (a -> Bool) -> [a] -> Bool
all :: (a -> Bool) -> [a] -> Bool
sum :: Num a => [a] -> a
product :: Num a => [a] -> a
concat :: [[a]] -> [a]
concatMap :: (a -> [b]) -> [a] -> [b]
maximum :: Ord a => [a] -> a
minimum :: Ord a => [a] -> a
scanl :: (a -> b -> a) -> a -> [b] -> [a]
scanl1 :: (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 -> [a] -> [a]
drop :: Int -> [a] -> [a]
splitAt :: Int -> [a] -> ([a], [a])
takeWhile :: (a -> Bool) -> [a] -> [a]
dropWhile :: (a -> Bool) -> [a] -> [a]
span :: (a -> Bool) -> [a] -> ([a], [a])
break :: (a -> Bool) -> [a] -> ([a], [a])
elem :: Eq a => a -> [a] -> Bool
notElem :: Eq a => a -> [a] -> Bool
lookup :: Eq a => a -> [(a, b)] -> Maybe 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
type ReadS a = String -> [(a, String)]
type ShowS = String -> String
class Read a where
readsPrec :: Int -> ReadS a
readList :: ReadS [a]
class Show a where
showsPrec :: Int -> a -> ShowS
show :: a -> String
showList :: [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
data IO a
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 ()
type FilePath = String
readFile :: FilePath -> IO String
writeFile :: FilePath -> String -> IO ()
appendFile :: FilePath -> String -> IO ()
readIO :: Read a => String -> IO a
readLn :: Read a => IO a
type IOError = IOException
ioError :: IOError -> IO a
userError :: String -> IOError
catch :: IO a -> (IOError -> IO a) -> IO a
Standard types, classes and related functions
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
Data Bool
Typeable Bool
Storable Bool
Ix Bool
Eq Bool
Ord Bool
Bounded Bool
Enum Bool
Read Bool
Show Bool
Random Bool
(&&) :: Bool -> Bool -> Bool
Boolean "and"
(||) :: Bool -> Bool -> Bool
Boolean "or"
not :: Bool -> Bool
Boolean "not"
otherwise :: Bool

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

  f x | x < 0     = ...
      | otherwise = ...
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
Data a => Data (Maybe a)
Functor Maybe
Monad Maybe
Eq a => Eq (Maybe a)
Ord a => Ord (Maybe a)
Typeable a => Typeable (Maybe a)
Read a => Read (Maybe a)
Show a => Show (Maybe a)
maybe :: b -> (a -> b) -> Maybe a -> b
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 that.
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)
(Eq a, Eq b) => Eq (Either a b)
(Ord a, Ord b) => Ord (Either a b)
(Data a, Data b) => Data (Either a b)
(Typeable a, Typeable b) => Typeable (Either a b)
(Read a, Read b) => Read (Either a b)
(Show a, Show b) => Show (Either a b)
either :: (a -> c) -> (b -> c) -> Either a b -> c
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.
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
Eq Ordering
Ord Ordering
Bounded Ordering
Enum Ordering
Read Ordering
Show Ordering
data Char

The character type Char is an enumeration whose values represent Unicode (or equivalently ISO 10646) characters. This set extends the ISO 8859-1 (Latin-1) character set (the first 256 charachers), which is itself an extension of the ASCII character set (the first 128 characters). A character literal in Haskell has type Char.

To convert a Char to or from the corresponding Int value defined by Unicode, use toEnum and fromEnum from the Enum class respectively (or equivalently ord and chr).

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
Data Char
Typeable Char
Storable Char
Ix Char
Eq Char
Ord Char
Bounded Char
Enum Char
Read Char
Show Char
Random Char
HTML Char
type String = [Char]
A String is a list of characters. String constants in Haskell are values of type String.
Tuples
fst :: (a, b) -> a
snd :: (a, b) -> b
curry :: ((a, b) -> c) -> a -> b -> c
uncurry :: (a -> b -> c) -> (a, b) -> c
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)
(RealFloat a, Eq a) => Eq (Complex a)
(Eq a, Eq b) => Eq (Either a b)
(Eq key, Eq elt) => Eq (FiniteMap key elt)
Eq Constr
Eq Fixity
Eq a => Eq (Maybe a)
Eq PackedString
Eq a => Eq (Set a)
(Eq a, Eq b) => Eq (a, b)
(Eq a, Eq b, Eq c) => Eq (a, b, c)
(Eq a, Eq b, Eq c, Eq d) => Eq (a, b, c, d)
(Eq a, Eq b, Eq c, Eq d, Eq e) => Eq (a, b, c, d, e)
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f) => Eq (a, b, c, d, e, f)
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g) => Eq (a, b, c, d, e, f, g)
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h) => Eq (a, b, c, d, e, f, g, h)
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i) => Eq (a, b, c, d, e, f, g, h, i)
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i, Eq j) => Eq (a, b, c, d, e, f, g, h, i, j)
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i, Eq j, Eq k) => Eq (a, b, c, d, e, f, g, h, i, j, k)
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i, Eq j, Eq k, Eq l) => Eq (a, b, c, d, e, f, g, h, i, j, k, l)
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i, Eq j, Eq k, Eq l, Eq m) => Eq (a, b, c, d, e, f, g, h, i, j, k, l, m)
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i, Eq j, Eq k, Eq l, Eq m, Eq n) => Eq (a, b, c, d, e, f, g, h, i, j, k, l, m, n)
(Eq a, Eq b, Eq c, Eq d, Eq e, Eq f, Eq g, Eq h, Eq i, Eq j, Eq k, Eq l, Eq m, Eq n, Eq o) => Eq (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)
Eq TypeRep
Eq TyCon
Eq Key
Eq KeyPr
Eq Unique
Eq Errno
Eq CChar
Eq CSChar
Eq CUChar
Eq CShort
Eq CUShort
Eq CInt
Eq CUInt
Eq CLong
Eq CULong
Eq CLLong
Eq CULLong
Eq CFloat
Eq CDouble
Eq CLDouble
Eq CPtrdiff
Eq CSize
Eq CWchar
Eq CSigAtomic
Eq CClock
Eq CTime
Eq (STArray s i e)
(Ix i, Eq e) => Eq (Array i e)
Eq a => Eq [a]
Eq ()
Eq Char
Eq Int
Eq Bool
Eq Ordering
Eq Float
Eq Double
Eq (ForeignPtr a)
Eq HandlePosn
Eq SeekMode
Eq (MVar a)
Eq Handle
Eq (IORef a)
Eq (IOArray i e)
Eq Exception
Eq IOException
Eq IOErrorType
Eq BufferState
Eq BufferMode
Eq ArithException
Eq AsyncException
Eq ArrayException
Eq ExitCode
Eq IOMode
Eq Int64
Eq Int8
Eq Int16
Eq Int32
Eq Integer
Eq (Ptr a)
Eq (FunPtr a)
(Integral a, Eq a) => Eq (Ratio a)
Eq (STRef s a)
Eq (StablePtr a)
Eq Word64
Eq Word
Eq Word8
Eq Word16
Eq Word32
Eq Permissions
Eq TimeLocale
Eq (StableName a)
Eq FDType
Eq CDev
Eq CIno
Eq CMode
Eq COff
Eq CPid
Eq CSsize
Eq CGid
Eq CNlink
Eq CUid
Eq CCc
Eq CSpeed
Eq CTcflag
Eq CRLim
Eq Fd
Eq Month
Eq Day
Eq ClockTime
Eq CalendarTime
Eq TimeDiff
Eq Lexeme
class Eq a => Ord a where
Methods
compare :: a -> a -> Ordering
(<) :: a -> a -> Bool
(<=) :: a -> a -> Bool
(>) :: a -> a -> Bool
(>=) :: a -> a -> Bool
max :: a -> a -> a
min :: a -> a -> a
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 a, Ord b) => Ord (Either a b)
Ord a => Ord (Maybe a)
Ord PackedString
(Ord a, Ord b) => Ord (a, b)
(Ord a, Ord b, Ord c) => Ord (a, b, c)
(Ord a, Ord b, Ord c, Ord d) => Ord (a, b, c, d)
(Ord a, Ord b, Ord c, Ord d, Ord e) => Ord (a, b, c, d, e)
(Ord a, Ord b, Ord c, Ord d, Ord e, Ord f) => Ord (a, b, c, d, e, f)
(Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g) => Ord (a, b, c, d, e, f, g)
(Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h) => Ord (a, b, c, d, e, f, g, h)
(Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i) => Ord (a, b, c, d, e, f, g, h, i)
(Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i, Ord j) => Ord (a, b, c, d, e, f, g, h, i, j)
(Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i, Ord j, Ord k) => Ord (a, b, c, d, e, f, g, h, i, j, k)
(Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i, Ord j, Ord k, Ord l) => Ord (a, b, c, d, e, f, g, h, i, j, k, l)
(Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i, Ord j, Ord k, Ord l, Ord m) => Ord (a, b, c, d, e, f, g, h, i, j, k, l, m)
(Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i, Ord j, Ord k, Ord l, Ord m, Ord n) => Ord (a, b, c, d, e, f, g, h, i, j, k, l, m, n)
(Ord a, Ord b, Ord c, Ord d, Ord e, Ord f, Ord g, Ord h, Ord i, Ord j, Ord k, Ord l, Ord m, Ord n, Ord o) => Ord (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)
Ord Unique
Ord CChar
Ord CSChar
Ord CUChar
Ord CShort
Ord CUShort
Ord CInt
Ord CUInt
Ord CLong
Ord CULong
Ord CLLong
Ord CULLong
Ord CFloat
Ord CDouble
Ord CLDouble
Ord CPtrdiff
Ord CSize
Ord CWchar
Ord CSigAtomic
Ord CClock
Ord CTime
(Ix i, Ord e) => Ord (Array i e)
Ord a => Ord [a]
Ord ()
Ord Char
Ord Int
Ord Bool
Ord Ordering
Ord Float
Ord Double
Ord (ForeignPtr a)
Ord SeekMode
Ord BufferMode
Ord ArithException
Ord AsyncException
Ord ArrayException
Ord ExitCode
Ord IOMode
Ord Int64
Ord Int8
Ord Int16
Ord Int32
Ord Integer
Ord (Ptr a)
Ord (FunPtr a)
Integral a => Ord (Ratio a)
Ord Word64
Ord Word
Ord Word8
Ord Word16
Ord Word32
Ord Permissions
Ord TimeLocale
Ord CDev
Ord CIno
Ord CMode
Ord COff
Ord CPid
Ord CSsize
Ord CGid
Ord CNlink
Ord CUid
Ord CCc
Ord CSpeed
Ord CTcflag
Ord CRLim
Ord Fd
Ord Month
Ord Day
Ord ClockTime
Ord CalendarTime
Ord TimeDiff
class Enum a where
Methods
succ :: a -> a
pred :: a -> a
toEnum :: Int -> a
fromEnum :: a -> Int
enumFrom :: a -> [a]
enumFromThen :: a -> a -> [a]
enumFromTo :: a -> a -> [a]
enumFromThenTo :: a -> a -> a -> [a]
Instances
Enum CChar
Enum CSChar
Enum CUChar
Enum CShort
Enum CUShort
Enum CInt
Enum CUInt
Enum CLong
Enum CULong
Enum CLLong
Enum CULLong
Enum CFloat
Enum CDouble
Enum CLDouble
Enum CPtrdiff
Enum CSize
Enum CWchar
Enum CSigAtomic
Enum CClock
Enum CTime
Enum ()
Enum Bool
Enum Ordering
Enum Char
Enum Int
Enum Float
Enum Double
Enum SeekMode
Enum IOMode
Enum Int8
Enum Int16
Enum Int32
Enum Int64
Enum Integer
Integral a => Enum (Ratio a)
Enum Word
Enum Word8
Enum Word16
Enum Word32
Enum Word64
Enum CDev
Enum CIno
Enum CMode
Enum COff
Enum CPid
Enum CSsize
Enum CGid
Enum CNlink
Enum CUid
Enum CCc
Enum CSpeed
Enum CTcflag
Enum CRLim
Enum Fd
Enum Month
Enum Day
class Bounded a where
Methods
minBound :: a
maxBound :: a
Instances
Bounded CChar
Bounded CSChar
Bounded CUChar
Bounded CShort
Bounded CUShort
Bounded CInt
Bounded CUInt
Bounded CLong
Bounded CULong
Bounded CLLong
Bounded CULLong
Bounded CPtrdiff
Bounded CSize
Bounded CWchar
Bounded CSigAtomic
Bounded CClock
Bounded CTime
Bounded ()
(Bounded a, Bounded b) => Bounded (a, b)
(Bounded a, Bounded b, Bounded c) => Bounded (a, b, c)
(Bounded a, Bounded b, Bounded c, Bounded d) => Bounded (a, b, c, d)
Bounded Bool
Bounded Ordering
Bounded Char
Bounded Int
Bounded Int8
Bounded Int16
Bounded Int32
Bounded Int64
Bounded Word
Bounded Word8
Bounded Word16
Bounded Word32
Bounded Word64
Bounded CIno
Bounded CMode
Bounded COff
Bounded CPid
Bounded CSsize
Bounded CGid
Bounded CNlink
Bounded CUid
Bounded CTcflag
Bounded CRLim
Bounded Fd
Bounded Month
Bounded Day
Numbers
Numeric types
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
Data Int
Typeable Int
Storable Int
Ix Int
Eq Int
Ord Int
Bounded Int
Enum Int
Num Int
Read Int
Real Int
Integral Int
Show Int
Random Int
data Integer
Arbitrary-precision integers.
Instances
Bits Integer
Data Integer
Typeable 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
Data Float
Typeable Float
Storable Float
Eq Float
Ord Float
Num Float
Real Float
Fractional Float
RealFrac Float
Floating Float
RealFloat Float
Show Float
Enum Float
Read 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
Storable Double
Eq Double
Ord Double
Num Double
Real Double
Fractional Double
Floating Double
RealFrac Double
RealFloat Double
Show Double
Enum Double
Read Double
Random Double
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.
Numeric type classes
class (Eq a, Show a) => Num a where
Methods
(+) :: a -> a -> a
(-) :: a -> a -> a
(*) :: a -> a -> a
negate :: a -> a
abs :: a -> a
signum :: a -> a
fromInteger :: Integer -> a
Instances
RealFloat a => Num (Complex a)
Num CChar
Num CSChar
Num CUChar
Num CShort
Num CUShort
Num CInt
Num CUInt
Num CLong
Num CULong
Num CLLong
Num CULLong
Num CFloat
Num CDouble
Num CLDouble
Num CPtrdiff
Num CSize
Num CWchar
Num CSigAtomic
Num CClock
Num CTime
Num Float
Num Double
Num Int8
Num Int16
Num Int32
Num Int64
Num Int
Num Integer
Integral a => Num (Ratio a)
Num Word
Num Word8
Num Word16
Num Word32
Num Word64
Num CDev
Num CIno
Num CMode
Num COff
Num CPid
Num CSsize
Num CGid
Num CNlink
Num CUid
Num CCc
Num CSpeed
Num CTcflag
Num CRLim
Num Fd
class (Num a, Ord a) => Real a where
Methods
toRational :: a -> Rational
Instances
Real CChar
Real CSChar
Real CUChar
Real CShort
Real CUShort
Real CInt
Real CUInt
Real CLong
Real CULong
Real CLLong
Real CULLong
Real CFloat
Real CDouble
Real CLDouble
Real CPtrdiff
Real CSize
Real CWchar
Real CSigAtomic
Real CClock
Real CTime
Real Float
Real Double
Real Int8
Real Int16
Real Int32
Real Int64
Real Int
Real Integer
Integral a => Real (Ratio a)
Real Word
Real Word8
Real Word16
Real Word32
Real Word64
Real CIno
Real CMode
Real COff
Real CPid
Real CSsize
Real CGid
Real CNlink
Real CUid
Real CTcflag
Real CRLim
Real Fd
class (Real a, Enum a) => Integral a where
Methods
quot :: a -> a -> a
rem :: a -> a -> a
div :: a -> a -> a
mod :: a -> a -> a
quotRem :: a -> a -> (a, a)
divMod :: a -> a -> (a, a)
toInteger :: a -> Integer
Instances
Integral CChar
Integral CSChar
Integral CUChar
Integral CShort
Integral CUShort
Integral CInt
Integral CUInt
Integral CLong
Integral CULong
Integral CLLong
Integral CULLong
Integral CPtrdiff
Integral CSize
Integral CWchar
Integral CSigAtomic
Integral CClock
Integral CTime
Integral Int8
Integral Int16
Integral Int32
Integral Int64
Integral Int
Integral Integer
Integral Word
Integral Word8
Integral Word16
Integral Word32
Integral Word64
Integral CIno
Integral CMode
Integral COff
Integral CPid
Integral CSsize
Integral CGid
Integral CNlink
Integral CUid
Integral CTcflag
Integral CRLim
Integral Fd
class Num a => Fractional a where
Methods
(/) :: a -> a -> a
recip :: a -> a
fromRational :: Rational -> a
Instances
RealFloat a => Fractional (Complex a)
Fractional CFloat
Fractional CDouble
Fractional CLDouble
Fractional Float
Fractional Double
Integral a => Fractional (Ratio a)
class Fractional a => Floating a where
Methods
pi :: a
exp :: a -> a
log :: a -> a
sqrt :: a -> a
(**) :: a -> a -> a
logBase :: a -> a -> a
sin :: a -> a
cos :: a -> a
tan :: a -> a
asin :: a -> a
acos :: a -> a
atan :: a -> a
sinh :: a -> a
cosh :: a -> a
tanh :: a -> a
asinh :: a -> a
acosh :: a -> a
atanh :: a -> a
Instances
RealFloat a => Floating (Complex a)
Floating CFloat
Floating CDouble
Floating CLDouble
Floating Float
Floating Double
class (Real a, Fractional a) => RealFrac a where
Methods
properFraction :: Integral b => a -> (b, a)
truncate :: Integral b => a -> b
round :: Integral b => a -> b
ceiling :: Integral b => a -> b
floor :: Integral b => a -> b
Instances
RealFrac CFloat
RealFrac CDouble
RealFrac CLDouble
RealFrac Float
RealFrac Double
Integral a => RealFrac (Ratio a)
class (RealFrac a, Floating a) => RealFloat a where
Methods
floatRadix :: a -> Integer
floatDigits :: a -> Int
floatRange :: a -> (Int, Int)
decodeFloat :: a -> (Integer, Int)
encodeFloat :: Integer -> Int -> a
exponent :: a -> Int
significand :: a -> a
scaleFloat :: Int -> a -> a
isNaN :: a -> Bool
isInfinite :: a -> Bool
isDenormalized :: a -> Bool
isNegativeZero :: a -> Bool
isIEEE :: a -> Bool
atan2 :: a -> a -> a
Instances
RealFloat CFloat
RealFloat CDouble
RealFloat CLDouble
RealFloat Float
RealFloat Double
Numeric functions
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
Monads and functors
class Monad m where

The Monad class defines the basic operations over a monad. Instances of Monad should satisfy the following laws: 

 return a >>= k  ==  k a
 m >>= return  ==  m
 m >>= (\x -> k x >>= h)  ==  (m >>= k) >>= h

Instances of both Monad and Functor should additionally satisfy the law: 

 fmap f xs  ==  xs >>= return . f

The instances of Monad for lists, Maybe and IO defined in the Prelude satisfy these laws.

Methods
(>>=) :: m a -> (a -> m b) -> m b
(>>) :: m a -> m b -> m b
return :: a -> m a
fail :: String -> m a
Instances
ArrowApply a => Monad (ArrowMonad a)
Monad (Cont r)
Monad m => Monad (ContT r m)
Error e => Monad (Either e)
(Monad m, Error e) => Monad (ErrorT e m)
Monad Identity
Monad m => Monad (ListT m)
Monoid w => Monad (RWS r w s)
(Monoid w, Monad m) => Monad (RWST r w s m)
Monad ((->) r)
Monad (Reader r)
Monad m => Monad (ReaderT r m)
Monad (ST s)
Monad (State s)
Monad m => Monad (StateT s m)
Monoid w => Monad (Writer w)
(Monoid w, Monad m) => Monad (WriterT w m)
Monad Maybe
Monad []
Monad IO
Monad (ST s)
Monad P
Monad ReadP
Monad ReadPrec
class Functor f where

The Functor class is used for types that can be mapped over. Instances of Functor should satisfy the following laws: 

 fmap id  ==  id
 fmap (f . g)  ==  fmap f . fmap g

The instances of Functor for lists, Maybe and IO defined in the Prelude satisfy these laws.

Methods
fmap :: (a -> b) -> f a -> f b
Instances
Functor (Cont r)
Monad m => Functor (ContT r m)
Functor (Either e)
Monad m => Functor (ErrorT e m)
Functor Identity
Monad m => Functor (ListT m)
Functor (RWS r w s)
Monad m => Functor (RWST r w s m)
Functor ((->) r)
Functor (Reader r)
Monad m => Functor (ReaderT r m)
Functor (ST s)
Functor (State s)
Monad m => Functor (StateT s m)
Functor (Writer w)
Monad m => Functor (WriterT w m)
Functor Maybe
Ix i => Functor (Array i)
Functor []
Functor IO
Functor (ST s)
Functor ReadP
Functor ReadPrec
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
Miscellaneous functions
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
List operations
map :: (a -> b) -> [a] -> [b]

map f xs is the list obtained by applying f to each element of xs, i.e., 

 map f [x1, x2, ..., xn] == [f x1, f x2, ..., f xn]
 map f [x1, x2, ...] == [f x1, f x2, ...]
(++) :: [a] -> [a] -> [a]

Append two lists, i.e., 

 [x1, ..., xm] ++ [y1, ..., yn] == [x1, ..., xm, y1, ..., yn]
 [x1, ..., xm] ++ [y1, ...] == [x1, ..., xm, y1, ...]

If the first list is not finite, the result is the first list.

filter :: (a -> Bool) -> [a] -> [a]

filter, applied to a predicate and a list, returns the list of those elements that satisfy the predicate; i.e., 

 filter p xs = [ x | x <- xs, p x]
head :: [a] -> a
Extract the first element of a list, which must be non-empty.
last :: [a] -> a
Extract the last element of a list, which must be finite and non-empty.
tail :: [a] -> [a]
Extract the elements after the head of a list, which must be non-empty.
init :: [a] -> [a]
Return all the elements of a list except the last one. The list must be finite and non-empty.
null :: [a] -> Bool
Test whether a list is empty.
length :: [a] -> Int
length returns the length of a finite list as an Int. It is an instance of the more general genericLength, the result type of which may be any kind of number.
(!!) :: [a] -> Int -> a
List index (subscript) operator, starting from 0. It is an instance of the more general genericIndex, which takes an index of any integral type.
reverse :: [a] -> [a]
reverse xs returns the elements of xs in reverse order. xs must be finite.
Reducing lists (folds)
foldl :: (a -> b -> a) -> a -> [b] -> a

foldl, applied to a binary operator, a starting value (typically the left-identity of the operator), and a list, reduces the list using the binary operator, from left to right: 

 foldl f z [x1, x2, ..., xn] == (...((z `f` x1) `f` x2) `f`...) `f` xn

The list must be finite.

foldl1 :: (a -> a -> a) -> [a] -> a
foldl1 is a variant of foldl that has no starting value argument, and thus must be applied to non-empty lists.
foldr :: (a -> b -> b) -> b -> [a] -> b

foldr, applied to a binary operator, a starting value (typically the right-identity of the operator), and a list, reduces the list using the binary operator, from right to left: 

 foldr f z [x1, x2, ..., xn] == x1 `f` (x2 `f` ... (xn `f` z)...)
foldr1 :: (a -> a -> a) -> [a] -> a
foldr1 is a variant of foldr that has no starting value argument, and thus must be applied to non-empty lists.
Special folds
and :: [Bool] -> Bool
and returns the conjunction of a Boolean list. For the result to be True, the list must be finite; False, however, results from a False value at a finite index of a finite or infinite list.
or :: [Bool] -> Bool
or returns the disjunction of a Boolean list. For the result to be False, the list must be finite; True, however, results from a True value at a finite index of a finite or infinite list.
any :: (a -> Bool) -> [a] -> Bool
Applied to a predicate and a list, any determines if any element of the list satisfies the predicate.
all :: (a -> Bool) -> [a] -> Bool
Applied to a predicate and a list, all determines if all elements of the list satisfy the predicate.
sum :: Num a => [a] -> a
The sum function computes the sum of a finite list of numbers.
product :: Num a => [a] -> a
The product function computes the product of a finite list of numbers.
concat :: [[a]] -> [a]
Concatenate a list of lists.
concatMap :: (a -> [b]) -> [a] -> [b]
Map a function over a list and concatenate the results.
maximum :: Ord a => [a] -> a
maximum returns the maximum value from a list, which must be non-empty, finite, and of an ordered type. It is a special case of maximumBy, which allows the programmer to supply their own comparison function.
minimum :: Ord a => [a] -> a
minimum returns the minimum value from a list, which must be non-empty, finite, and of an ordered type. It is a special case of minimumBy, which allows the programmer to supply their own comparison function.
Building lists
Scans
scanl :: (a -> b -> a) -> a -> [b] -> [a]

scanl is similar to foldl, but returns a list of successive reduced values from the left: 

 scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]

Note that 

 last (scanl f z xs) == foldl f z xs.
scanl1 :: (a -> a -> a) -> [a] -> [a]

scanl1 is a variant of scanl that has no starting value argument: 

 scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]
scanr :: (a -> b -> b) -> b -> [a] -> [b]

scanr is the right-to-left dual of scanl. Note that 

 head (scanr f z xs) == foldr f z xs.
scanr1 :: (a -> a -> a) -> [a] -> [a]
scanr1 is a variant of scanr that has no starting value argument.
Infinite lists
iterate :: (a -> a) -> a -> [a]

iterate f x returns an infinite list of repeated applications of f to x: 

 iterate f x == [x, f x, f (f x), ...]
repeat :: a -> [a]
repeat x is an infinite list, with x the value of every element.
replicate :: Int -> a -> [a]
replicate n x is a list of length n with x the value of every element. It is an instance of the more general genericReplicate, in which n may be of any integral type.
cycle :: [a] -> [a]
cycle ties a finite list into a circular one, or equivalently, the infinite repetition of the original list. It is the identity on infinite lists.
Sublists
take :: Int -> [a] -> [a]
take n, applied to a list xs, returns the prefix of xs of length n, or xs itself if n > length xs. It is an instance of the more general genericTake, in which n may be of any integral type.
drop :: Int -> [a] -> [a]
drop n xs returns the suffix of xs after the first n elements, or [] if n > length xs. It is an instance of the more general genericDrop, in which n may be of any integral type.
splitAt :: Int -> [a] -> ([a], [a])
splitAt n xs is equivalent to (take n xs, drop n xs). It is an instance of the more general genericSplitAt, in which n may be of any integral type.
takeWhile :: (a -> Bool) -> [a] -> [a]
takeWhile, applied to a predicate p and a list xs, returns the longest prefix (possibly empty) of xs of elements that satisfy p.
dropWhile :: (a -> Bool) -> [a] -> [a]
dropWhile p xs returns the suffix remaining after takeWhile p xs.
span :: (a -> Bool) -> [a] -> ([a], [a])
span p xs is equivalent to (takeWhile p xs, dropWhile p xs)
break :: (a -> Bool) -> [a] -> ([a], [a])
break p is equivalent to span (not . p).
Searching lists
elem :: Eq a => a -> [a] -> Bool
elem is the list membership predicate, usually written in infix form, e.g., x elem xs.
notElem :: Eq a => a -> [a] -> Bool
notElem is the negation of elem.
lookup :: Eq a => a -> [(a, b)] -> Maybe b
lookup key assocs looks up a key in an association list.
Zipping and unzipping lists
zip :: [a] -> [b] -> [(a, b)]
zip takes two lists and returns a list of corresponding pairs. If one input list is short, excess elements of the longer list are discarded.
zip3 :: [a] -> [b] -> [c] -> [(a, b, c)]
zip3 takes three lists and returns a list of triples, analogous to zip.
zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
zipWith generalises zip by zipping with the function given as the first argument, instead of a tupling function. For example, zipWith (+) is applied to two lists to produce the list of corresponding sums.
zipWith3 :: (a -> b -> c -> d) -> [a] -> [b] -> [c] -> [d]
The zipWith3 function takes a function which combines three elements, as well as three lists and returns a list of their point-wise combination, analogous to zipWith.
unzip :: [(a, b)] -> ([a], [b])
unzip transforms a list of pairs into a list of first components and a list of second components.
unzip3 :: [(a, b, c)] -> ([a], [b], [c])
The unzip3 function takes a list of triples and returns three lists, analogous to unzip.
Functions on strings
lines :: String -> [String]
lines breaks a string up into a list of strings at newline characters. The resulting strings do not contain newlines.
words :: String -> [String]
words breaks a string up into a list of words, which were delimited by white space.
unlines :: [String] -> String
unlines is an inverse operation to lines. It joins lines, after appending a terminating newline to each.
unwords :: [String] -> String
unwords is an inverse operation to words. It joins words with separating spaces.
Converting to and from String
type ReadS a = String -> [(a, String)]
A parser for a type a, represented as a function that takes a String and returns a list of possible parses (a,String) pairs.
type ShowS = String -> String
class Read a where
Methods
readsPrec :: Int -> ReadS a
readList :: ReadS [a]
Instances
(RealFloat a, Read a) => Read (Complex a)
Read CChar
Read CSChar
Read CUChar
Read CShort
Read CUShort
Read CInt
Read CUInt
Read CLong
Read CULong
Read CLLong
Read CULLong
Read CFloat
Read CDouble
Read CLDouble
Read CPtrdiff
Read CSize
Read CWchar
Read CSigAtomic
Read CClock
Read CTime
Read SeekMode
Read BufferMode
Read ExitCode
Read IOMode
Read Int8
Read Int16
Read Int32
Read Int64
Read Char
Read Bool
Read Ordering
Read a => Read (Maybe a)
(Read a, Read b) => Read (Either a b)
Read a => Read [a]
(Ix a, Read a, Read b) => Read (Array a b)
Read Lexeme
Read Int
Read Integer
Read Float
Read Double
(Integral a, Read a) => Read (Ratio a)
Read ()
(Read a, Read b) => Read (a, b)
(Read a, Read b, Read c) => Read (a, b, c)
(Read a, Read b, Read c, Read d) => Read (a, b, c, d)
(Read a, Read b, Read c, Read d, Read e) => Read (a, b, c, d, e)
Read Word
Read Word8
Read Word16
Read Word32
Read Word64
Read Permissions
Read CDev
Read CIno
Read CMode
Read COff
Read CPid
Read CSsize
Read CGid
Read CNlink
Read CUid
Read CCc
Read CSpeed
Read CTcflag
Read CRLim
Read Fd
Read StdGen
Read Month
Read Day
Read CalendarTime
Read TimeDiff
class Show a where
Methods
showsPrec :: Int -> a -> ShowS
show :: a -> String
showList :: [a] -> ShowS
Instances
Show ThreadId
(Ix ix, Show ix) => Show (UArray ix Bool)
(Ix ix, Show ix) => Show (UArray ix Char)
(Ix ix, Show ix) => Show (UArray ix Int)
(Ix ix, Show ix) => Show (UArray ix Word)
(Ix ix, Show ix) => Show (UArray ix Float)
(Ix ix, Show ix) => Show (UArray ix Double)
(Ix ix, Show ix) => Show (UArray ix Int8)
(Ix ix, Show ix) => Show (UArray ix Int16)
(Ix ix, Show ix) => Show (UArray ix Int32)
(Ix ix, Show ix) => Show (UArray ix Int64)
(Ix ix, Show ix) => Show (UArray ix Word8)
(Ix ix, Show ix) => Show (UArray ix Word16)
(Ix ix, Show ix) => Show (UArray ix Word32)
(Ix ix, Show ix) => Show (UArray ix Word64)
(Ix ix, Show ix, Show e) => Show (DiffArray ix e)
(Ix ix, Show ix) => Show (DiffUArray ix Char)
(Ix ix, Show ix) => Show (DiffUArray ix Int)
(Ix ix, Show ix) => Show (DiffUArray ix Word)
(Ix ix, Show ix) => Show (DiffUArray ix Float)
(Ix ix, Show ix) => Show (DiffUArray ix Double)
(Ix ix, Show ix) => Show (DiffUArray ix Int8)
(Ix ix, Show ix) => Show (DiffUArray ix Int16)
(Ix ix, Show ix) => Show (DiffUArray ix Int32)
(Ix ix, Show ix) => Show (DiffUArray ix Int64)
(Ix ix, Show ix) => Show (DiffUArray ix Word8)
(Ix ix, Show ix) => Show (DiffUArray ix Word16)
(Ix ix, Show ix) => Show (DiffUArray ix Word32)
(Ix ix, Show ix) => Show (DiffUArray ix Word64)
(RealFloat a, Show a) => Show (Complex a)
Show Dynamic
Show Constr
Show Fixity
Show DataType
Show PackedString
Show TypeRep
Show TyCon
Show CChar
Show CSChar
Show CUChar
Show CShort
Show CUShort
Show CInt
Show CUInt
Show CLong
Show CULong
Show CLLong
Show CULLong
Show CFloat
Show CDouble
Show CLDouble
Show CPtrdiff
Show CSize
Show CWchar
Show CSigAtomic
Show CClock
Show CTime
Show (Ptr a)
Show (FunPtr a)
(Ix a, Show a, Show b) => Show (Array a b)
Show Float
Show Double
Show (ForeignPtr a)
Show HandlePosn
Show SeekMode
Show HandleType
Show Handle
Show ArithException
Show AsyncException
Show ArrayException
Show Exception
Show IOErrorType
Show IOException
Show BufferMode
Show ExitCode
Show IOMode
Show Int8
Show Int16
Show Int32
Show Int64
Show Integer
Integral a => Show (Ratio a)
Show (ST s a)
Show ()
Show a => Show [a]
Show Bool
Show Ordering
Show Char
Show Int
Show a => Show (Maybe a)
(Show a, Show b) => Show (Either a b)
(Show a, Show b) => Show (a, b)
(Show a, Show b, Show c) => Show (a, b, c)
(Show a, Show b, Show c, Show d) => Show (a, b, c, d)
(Show a, Show b, Show c, Show d, Show e) => Show (a, b, c, d, e)
Show Word
Show Word8
Show Word16
Show Word32
Show Word64
Show Permissions
Show TimeLocale
Show CDev
Show CIno
Show CMode
Show COff
Show CPid
Show CSsize
Show CGid
Show CNlink
Show CUid
Show CCc
Show CSpeed
Show CTcflag
Show CRLim
Show Fd
Show StdGen
Show ClockTime
Show Month
Show Day
Show CalendarTime
Show TimeDiff
Show Html
Show HtmlAttr
Show HtmlTable
Show HotLink
Show a => Show (BlockTable a)
Show Doc
Show Lexeme
Show (a -> b)
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
Basic Input and output
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
Simple I/O operations
Output functions
putChar :: Char -> IO ()
Write a character to the standard output device (same as hPutChar stdout).
putStr :: String -> IO ()
Write a string to the standard output device (same as hPutStr stdout).
putStrLn :: String -> IO ()
The same as putStrLn, but adds a newline character.
print :: Show a => a -> IO ()

The print function outputs a value of any printable type to the standard output device. Printable types are those that are instances of class Show; print converts values to strings for output using the show operation and adds a newline.

For example, a program to print the first 20 integers and their powers of 2 could be written as: 

 main = print ([(n, 2^n) | n <- [0..19]])
Input functions
getChar :: IO Char
Read a character from the standard input device (same as hGetChar stdin).
getLine :: IO String
Read a line from the standard input device (same as hGetLine stdin).
getContents :: IO String
The getContents operation returns all user input as a single string, which is read lazily as it is needed (same as hGetContents stdin).
interact :: (String -> String) -> IO ()
The interact function takes a function of type String->String as its argument. The entire input from the standard input device is passed to this function as its argument, and the resulting string is output on the standard output device.
Files
type FilePath = String
File and directory names are values of type String, whose precise meaning is operating system dependent. Files can be opened, yielding a handle which can then be used to operate on the contents of that file.
readFile :: FilePath -> IO String
The readFile function reads a file and returns the contents of the file as a string. The file is read lazily, on demand, as with getContents.
writeFile :: FilePath -> String -> IO ()
The computation writeFile file str function writes the string str, to the file file.
appendFile :: FilePath -> String -> IO ()

The computation appendFile file str function appends the string str, to the file file.

Note that writeFile and appendFile write a literal string to a file. To write a value of any printable type, as with print, use the show function to convert the value to a string first. 

 main = appendFile "squares" (show [(x,x*x) | x <- [0,0.1..2]])
readIO :: Read a => String -> IO a
The readIO function is similar to read except that it signals parse failure to the IO monad instead of terminating the program.
readLn :: Read a => IO a
The readLn function combines getLine and readIO.
Exception handling in the I/O monad
type IOError = IOException

The Haskell 98 type for exceptions in the IO monad. Any I/O operation may raise an IOError instead of returning a result. For a more general type of exception, including also those that arise in pure code, see Exception.

In Haskell 98, this is an opaque type.

ioError :: IOError -> IO a
Raise an IOError in the IO monad.
userError :: String -> IOError

Construct an IOError value with a string describing the error. The fail method of the IO instance of the Monad class raises a userError, thus: 

 instance Monad IO where 
   ...
   fail s = ioError (userError s)
catch :: IO a -> (IOError -> IO a) -> IO a

The catch function establishes a handler that receives any IOError raised in the action protected by catch. An IOError is caught by the most recent handler established by catch. These handlers are not selective: all IOErrors are caught. Exception propagation must be explicitly provided in a handler by re-raising any unwanted exceptions. For example, in 

 f = catch g (\e -> if IO.isEOFError e then return [] else ioError e)

the function f returns [] when an end-of-file exception (cf. isEOFError) occurs in g; otherwise, the exception is propagated to the next outer handler.

When an exception propagates outside the main program, the Haskell system prints the associated IOError value and exits the program.

Non-I/O exceptions are not caught by this variant; to catch all exceptions, use catch from Control.Exception.

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