| base-4.1.0.0: Basic libraries | Source code | Contents | Index

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`
  Converting to `String`
  Converting 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

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
Ordering (LT, EQ, GT)
Char (C#)
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

Int (I#)
Integer (S#, J#)
Float (F#)
Double (D#)
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
sqrt :: a -> a
log :: a -> a
(**) :: a -> a -> a
logBase :: a -> a -> a
sin :: a -> a
tan :: a -> a
cos :: a -> a
asin :: a -> a
atan :: a -> a
acos :: a -> a
sinh :: a -> a
tanh :: a -> a
cosh :: a -> a
asinh :: a -> a
atanh :: a -> a
acosh :: 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

(>>=) :: forall a b. m a -> (a -> m b) -> m b
(>>) :: forall a 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
($) :: (a -> b) -> a -> b
until :: (a -> Bool) -> (a -> a) -> a -> a
asTypeOf :: a -> a -> a
error :: [Char] -> a
undefined :: a
seq
($!) :: (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 ShowS = String -> String
class Show a where

showsPrec :: Int -> a -> ShowS
show :: a -> String
showList :: [a] -> ShowS

shows :: Show a => a -> ShowS
showChar :: Char -> ShowS
showString :: String -> ShowS
showParen :: Bool -> ShowS -> ShowS
type ReadS a = String -> [(a, String)]
class Read a where

readsPrec :: Int -> ReadS a
readList :: ReadS [a]

reads :: Read a => ReadS a
readParen :: Bool -> ReadS a -> ReadS a
read :: Read a => String -> a
lex :: ReadS String
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
Bool (False, True)

(&&) :: Bool -> Bool -> Bool | Source

Boolean "and"

(||) :: Bool -> Bool -> Bool | Source

Boolean "or"

not :: Bool -> Bool | Source

Boolean "not"

otherwise :: Bool | Source

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

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

data Maybe a | Source

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 Data.Either.Either type.

Constructors

Nothing |
Just a |

Instances

Monad Maybe
Functor Maybe
Typeable1 Maybe
MonadFix Maybe
MonadPlus Maybe
Alternative Maybe
Applicative Maybe
Foldable Maybe
Traversable Maybe
Eq a => Eq (Maybe a)
Data a => Data (Maybe a)
Ord a => Ord (Maybe a)
Read a => Read (Maybe a)
Show a => Show (Maybe a)
Monoid a => Monoid (Maybe a)

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.

data Either a b | Source

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

Typeable2 Either
Functor (Either a)
(Eq a, Eq b) => Eq (Either a b)
(Data a, Data b) => Data (Either a b)
(Ord a, Ord b) => Ord (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 | 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`.
Ordering (LT, EQ, GT)
Char (C#)

type String = [Char] | Source

A `String` is a list of characters. String constants in Haskell are values of type `String`.
Tuples

fst :: (a, b) -> a | Source

Extract the first component of a pair.

snd :: (a, b) -> b | Source

Extract the second component of a pair.

curry :: ((a, b) -> c) -> a -> b -> c | Source

`curry` converts an uncurried function to a curried function.

uncurry :: (a -> b -> c) -> (a, b) -> c | Source

`uncurry` converts a curried function to a function on pairs.
Basic type classes

class Eq a where | Source

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 | Source

(/=) :: a -> a -> Bool | Source

Instances

Eq Bool
Eq Char
Eq Double
Eq Float
Eq Int
Eq Int8
Eq Int16
Eq Int32
Eq Int64
Eq Integer
Eq Ordering
Eq Word
Eq Word8
Eq Word16
Eq Word32
Eq Word64
Eq ()
Eq TyCon
Eq TypeRep
Eq ArithException
Eq Lexeme
Eq CUIntMax
Eq CIntMax
Eq CUIntPtr
Eq CIntPtr
Eq CTime
Eq CClock
Eq CSigAtomic
Eq CWchar
Eq CSize
Eq CPtrdiff
Eq CLDouble
Eq CDouble
Eq CFloat
Eq CULLong
Eq CLLong
Eq CULong
Eq CLong
Eq CUInt
Eq CInt
Eq CUShort
Eq CShort
Eq CUChar
Eq CSChar
Eq CChar
Eq IOMode
Eq IOErrorType
Eq IOException
Eq ExitCode
Eq ArrayException
Eq AsyncException
Eq BufferMode
Eq BufferState
Eq Handle
Eq GeneralCategory
Eq Inserts
Eq HashData
Eq KeyPr
Eq Key
Eq Finalizers
Eq IntPtr
Eq WordPtr
Eq Errno
Eq Fd
Eq CRLim
Eq CTcflag
Eq CSpeed
Eq CCc
Eq CUid
Eq CNlink
Eq CGid
Eq CSsize
Eq CPid
Eq COff
Eq CMode
Eq CIno
Eq CDev
Eq FDType
Eq ThreadStatus
Eq BlockReason
Eq ThreadId
Eq SeekMode
Eq HandlePosn
Eq Exception
Eq Fixity
Eq ConstrRep
Eq DataRep
Eq Constr
Eq Any
Eq All
Eq Unique
Eq Timeout
Eq Version
Eq a => Eq ([] a)
Integral a => Eq (Ratio a)
Eq (StablePtr a)
Eq (Ptr a)
Eq (FunPtr a)
Eq a => Eq ([::] a)
Eq a => Eq (Maybe a)
Eq (IORef a)
Eq (MVar a)
Eq (ForeignPtr a)
Eq (TVar a)
Eq a => Eq (Down a)
RealFloat a => Eq (Complex a)
Eq (Fixed a)
Eq a => Eq (Last a)
Eq a => Eq (First a)
Eq a => Eq (Product a)
Eq a => Eq (Sum a)
Eq a => Eq (Dual a)
Eq (StableName a)
(Eq a, Eq b) => Eq (Either a b)
(Eq a, Eq b) => Eq ((,) a b)
(Ix i, Eq e) => Eq (Array i e)
Eq (STRef s a)
Eq (IOArray i e)
(Eq a, Eq b, Eq c) => Eq ((,,) a b c)
Eq (STArray s i e)
(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)

class Eq a => Ord a where | Source

The `Ord` class is used for totally ordered datatypes.

Instances of `Ord` can be derived for any user-defined datatype whose constituent types are in `Ord`. The declared order of the constructors in the data declaration determines the ordering in derived `Ord` instances. The `Ordering` datatype allows a single comparison to determine the precise ordering of two objects.

Minimal complete definition: either `compare` or `<=`. Using `compare` can be more efficient for complex types.

Methods

compare :: a -> a -> Ordering | Source

(<) :: a -> a -> Bool | Source

(>=) :: a -> a -> Bool | Source

(>) :: a -> a -> Bool | Source

(<=) :: a -> a -> Bool | Source

max :: a -> a -> a | Source

min :: a -> a -> a | Source

Instances

Ord Bool
Ord Char
Ord Double
Ord Float
Ord Int
Ord Int8
Ord Int16
Ord Int32
Ord Int64
Ord Integer
Ord Ordering
Ord Word
Ord Word8
Ord Word16
Ord Word32
Ord Word64
Ord ()
Ord ArithException
Ord CUIntMax
Ord CIntMax
Ord CUIntPtr
Ord CIntPtr
Ord CTime
Ord CClock
Ord CSigAtomic
Ord CWchar
Ord CSize
Ord CPtrdiff
Ord CLDouble
Ord CDouble
Ord CFloat
Ord CULLong
Ord CLLong
Ord CULong
Ord CLong
Ord CUInt
Ord CInt
Ord CUShort
Ord CShort
Ord CUChar
Ord CSChar
Ord CChar
Ord IOMode
Ord ExitCode
Ord ArrayException
Ord AsyncException
Ord BufferMode
Ord GeneralCategory
Ord IntPtr
Ord WordPtr
Ord Fd
Ord CRLim
Ord CTcflag
Ord CSpeed
Ord CCc
Ord CUid
Ord CNlink
Ord CGid
Ord CSsize
Ord CPid
Ord COff
Ord CMode
Ord CIno
Ord CDev
Ord ThreadStatus
Ord BlockReason
Ord ThreadId
Ord SeekMode
Ord Any
Ord All
Ord Unique
Ord Version
Ord a => Ord ([] a)
Integral a => Ord (Ratio a)
Ord (Ptr a)
Ord (FunPtr a)
Ord a => Ord ([::] a)
Ord a => Ord (Maybe a)
Ord (ForeignPtr a)
Ord a => Ord (Down a)
Ord (Fixed a)
Ord a => Ord (Last a)
Ord a => Ord (First a)
Ord a => Ord (Product a)
Ord a => Ord (Sum a)
Ord a => Ord (Dual a)
(Ord a, Ord b) => Ord (Either a b)
(Ord a, Ord b) => Ord ((,) a b)
(Ix i, Ord e) => Ord (Array i e)
(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)

class Enum a where | Source

Class `Enum` defines operations on sequentially ordered types.

The `enumFrom`... methods are used in Haskell's translation of arithmetic sequences.

Instances of `Enum` may be derived for any enumeration type (types whose constructors have no fields). The nullary constructors are assumed to be numbered left-to-right by `fromEnum` from `0` through `n-1`. See Chapter 10 of the Haskell Report for more details.

For any type that is an instance of class `Bounded` as well as `Enum`, the following should hold:

- The calls ``succ` `maxBound`` and ``pred` `minBound`` should result in a runtime error.
- `fromEnum` and `toEnum` should give a runtime error if the result value is not representable in the result type. For example, ``toEnum` 7 :: `Bool`` is an error.
- `enumFrom` and `enumFromThen` should be defined with an implicit bound, thus:

        enumFrom     x   = enumFromTo     x maxBound
        enumFromThen x y = enumFromThenTo x y bound
          where
            bound | fromEnum y >= fromEnum x = maxBound
                  | otherwise                = minBound

Methods

succ :: a -> a | Source

the successor of a value. For numeric types, `succ` adds 1.

pred :: a -> a | Source

the predecessor of a value. For numeric types, `pred` subtracts 1.

toEnum :: Int -> a | Source

Convert from an `Int`.

fromEnum :: a -> Int | Source

Convert to an `Int`. It is implementation-dependent what `fromEnum` returns when applied to a value that is too large to fit in an `Int`.

enumFrom :: a -> [a] | Source

Used in Haskell's translation of `[n..]`.

enumFromThen :: a -> a -> [a] | Source

Used in Haskell's translation of `[n,n'..]`.

enumFromTo :: a -> a -> [a] | Source

Used in Haskell's translation of `[n..m]`.

enumFromThenTo :: a -> a -> a -> [a] | Source

Used in Haskell's translation of `[n,n'..m]`.

Instances

Enum Bool
Enum Char
Enum Double
Enum Float
Enum Int
Enum Int8
Enum Int16
Enum Int32
Enum Int64
Enum Integer
Enum Ordering
Enum Word
Enum Word8
Enum Word16
Enum Word32
Enum Word64
Enum ()
Enum CUIntMax
Enum CIntMax
Enum CUIntPtr
Enum CIntPtr
Enum CTime
Enum CClock
Enum CSigAtomic
Enum CWchar
Enum CSize
Enum CPtrdiff
Enum CLDouble
Enum CDouble
Enum CFloat
Enum CULLong
Enum CLLong
Enum CULong
Enum CLong
Enum CUInt
Enum CInt
Enum CUShort
Enum CShort
Enum CUChar
Enum CSChar
Enum CChar
Enum IOMode
Enum GeneralCategory
Enum IntPtr
Enum WordPtr
Enum Fd
Enum CRLim
Enum CTcflag
Enum CSpeed
Enum CCc
Enum CUid
Enum CNlink
Enum CGid
Enum CSsize
Enum CPid
Enum COff
Enum CMode
Enum CIno
Enum CDev
Enum SeekMode
Integral a => Enum (Ratio a)
Enum (Fixed a)

class Bounded a where | Source

The `Bounded` class is used to name the upper and lower limits of a type. `Ord` is not a superclass of `Bounded` since types that are not totally ordered may also have upper and lower bounds.

The `Bounded` class may be derived for any enumeration type; `minBound` is the first constructor listed in the `data` declaration and `maxBound` is the last. `Bounded` may also be derived for single-constructor datatypes whose constituent types are in `Bounded`.

Methods

minBound :: a | Source

maxBound :: a | Source

Instances

Bounded Bool
Bounded Char
Bounded Int
Bounded Int8
Bounded Int16
Bounded Int32
Bounded Int64
Bounded Ordering
Bounded Word
Bounded Word8
Bounded Word16
Bounded Word32
Bounded Word64
Bounded ()
Bounded CUIntMax
Bounded CIntMax
Bounded CUIntPtr
Bounded CIntPtr
Bounded CSigAtomic
Bounded CWchar
Bounded CSize
Bounded CPtrdiff
Bounded CULLong
Bounded CLLong
Bounded CULong
Bounded CLong
Bounded CUInt
Bounded CInt
Bounded CUShort
Bounded CShort
Bounded CUChar
Bounded CSChar
Bounded CChar
Bounded GeneralCategory
Bounded IntPtr
Bounded WordPtr
Bounded Fd
Bounded CRLim
Bounded CTcflag
Bounded CUid
Bounded CNlink
Bounded CGid
Bounded CSsize
Bounded CPid
Bounded COff
Bounded CMode
Bounded CIno
Bounded Any
Bounded All
Bounded a => Bounded (Product a)
Bounded a => Bounded (Sum a)
Bounded a => Bounded (Dual a)
(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 a, Bounded b, Bounded c, Bounded d, Bounded e) => Bounded ((,,,,) a b c d e)
(Bounded a, Bounded b, Bounded c, Bounded d, Bounded e, Bounded f) => Bounded ((,,,,,) a b c d e f)
(Bounded a, Bounded b, Bounded c, Bounded d, Bounded e, Bounded f, Bounded g) => Bounded ((,,,,,,) a b c d e f g)
(Bounded a, Bounded b, Bounded c, Bounded d, Bounded e, Bounded f, Bounded g, Bounded h) => Bounded ((,,,,,,,) a b c d e f g h)
(Bounded a, Bounded b, Bounded c, Bounded d, Bounded e, Bounded f, Bounded g, Bounded h, Bounded i) => Bounded ((,,,,,,,,) a b c d e f g h i)
(Bounded a, Bounded b, Bounded c, Bounded d, Bounded e, Bounded f, Bounded g, Bounded h, Bounded i, Bounded j) => Bounded ((,,,,,,,,,) a b c d e f g h i j)
(Bounded a, Bounded b, Bounded c, Bounded d, Bounded e, Bounded f, Bounded g, Bounded h, Bounded i, Bounded j, Bounded k) => Bounded ((,,,,,,,,,,) a b c d e f g h i j k)
(Bounded a, Bounded b, Bounded c, Bounded d, Bounded e, Bounded f, Bounded g, Bounded h, Bounded i, Bounded j, Bounded k, Bounded l) => Bounded ((,,,,,,,,,,,) a b c d e f g h i j k l)
(Bounded a, Bounded b, Bounded c, Bounded d, Bounded e, Bounded f, Bounded g, Bounded h, Bounded i, Bounded j, Bounded k, Bounded l, Bounded m) => Bounded ((,,,,,,,,,,,,) a b c d e f g h i j k l m)
(Bounded a, Bounded b, Bounded c, Bounded d, Bounded e, Bounded f, Bounded g, Bounded h, Bounded i, Bounded j, Bounded k, Bounded l, Bounded m, Bounded n) => Bounded ((,,,,,,,,,,,,,) a b c d e f g h i j k l m n)
(Bounded a, Bounded b, Bounded c, Bounded d, Bounded e, Bounded f, Bounded g, Bounded h, Bounded i, Bounded j, Bounded k, Bounded l, Bounded m, Bounded n, Bounded o) => Bounded ((,,,,,,,,,,,,,,) a b c d e f g h i j k l m n o)

Numbers
Numeric types
Int (I#)
Integer (S#, J#)
Float (F#)
Double (D#)

type Rational = Ratio Integer | Source

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 | Source

Basic numeric class. Minimal complete definition: all except `negate` or `(-)`
Methods

(+) :: a -> a -> a | Source

(*) :: a -> a -> a | Source

(-) :: a -> a -> a | Source

negate :: a -> a | Source

Unary negation.

abs :: a -> a | Source

Absolute value.

signum :: a -> a | Source

Sign of a number. The functions `abs` and `signum` should satisfy the law:

     abs x * signum x == x

For real numbers, the `signum` is either `-1` (negative), `0` (zero) or `1` (positive).

fromInteger :: Integer -> a | Source

Conversion from an `Integer`. An integer literal represents the application of the function `fromInteger` to the appropriate value of type `Integer`, so such literals have type `(`Num` a) => a`.

Instances

Num Double
Num Float
Num Int
Num Int8
Num Int16
Num Int32
Num Int64
Num Integer
Num Word
Num Word8
Num Word16
Num Word32
Num Word64
Num CUIntMax
Num CIntMax
Num CUIntPtr
Num CIntPtr
Num CTime
Num CClock
Num CSigAtomic
Num CWchar
Num CSize
Num CPtrdiff
Num CLDouble
Num CDouble
Num CFloat
Num CULLong
Num CLLong
Num CULong
Num CLong
Num CUInt
Num CInt
Num CUShort
Num CShort
Num CUChar
Num CSChar
Num CChar
Num IntPtr
Num WordPtr
Num Fd
Num CRLim
Num CTcflag
Num CSpeed
Num CCc
Num CUid
Num CNlink
Num CGid
Num CSsize
Num CPid
Num COff
Num CMode
Num CIno
Num CDev
Integral a => Num (Ratio a)
RealFloat a => Num (Complex a)
HasResolution a => Num (Fixed a)

class (Num a, Ord a) => Real a where | Source

Methods

toRational :: a -> Rational | Source

the rational equivalent of its real argument with full precision

Instances

Real Double
Real Float
Real Int
Real Int8
Real Int16
Real Int32
Real Int64
Real Integer
Real Word
Real Word8
Real Word16
Real Word32
Real Word64
Real CUIntMax
Real CIntMax
Real CUIntPtr
Real CIntPtr
Real CTime
Real CClock
Real CSigAtomic
Real CWchar
Real CSize
Real CPtrdiff
Real CLDouble
Real CDouble
Real CFloat
Real CULLong
Real CLLong
Real CULong
Real CLong
Real CUInt
Real CInt
Real CUShort
Real CShort
Real CUChar
Real CSChar
Real CChar
Real IntPtr
Real WordPtr
Real Fd
Real CRLim
Real CTcflag
Real CSpeed
Real CCc
Real CUid
Real CNlink
Real CGid
Real CSsize
Real CPid
Real COff
Real CMode
Real CIno
Real CDev
Integral a => Real (Ratio a)
HasResolution a => Real (Fixed a)

class (Real a, Enum a) => Integral a where | Source

Integral numbers, supporting integer division. Minimal complete definition: `quotRem` and `toInteger`
Methods

quot :: a -> a -> a | Source

integer division truncated toward zero

rem :: a -> a -> a | Source

integer remainder, satisfying

     (x `quot` y)*y + (x `rem` y) == x

div :: a -> a -> a | Source

integer division truncated toward negative infinity

mod :: a -> a -> a | Source

integer modulus, satisfying

     (x `div` y)*y + (x `mod` y) == x

quotRem :: a -> a -> (a, a) | Source

simultaneous `quot` and `rem`

divMod :: a -> a -> (a, a) | Source

simultaneous `div` and `mod`

toInteger :: a -> Integer | Source

conversion to `Integer`

Instances

Integral Int
Integral Int8
Integral Int16
Integral Int32
Integral Int64
Integral Integer
Integral Word
Integral Word8
Integral Word16
Integral Word32
Integral Word64
Integral CUIntMax
Integral CIntMax
Integral CUIntPtr
Integral CIntPtr
Integral CSigAtomic
Integral CWchar
Integral CSize
Integral CPtrdiff
Integral CULLong
Integral CLLong
Integral CULong
Integral CLong
Integral CUInt
Integral CInt
Integral CUShort
Integral CShort
Integral CUChar
Integral CSChar
Integral CChar
Integral IntPtr
Integral WordPtr
Integral Fd
Integral CRLim
Integral CTcflag
Integral CUid
Integral CNlink
Integral CGid
Integral CSsize
Integral CPid
Integral COff
Integral CMode
Integral CIno

class Num a => Fractional a where | Source

Fractional numbers, supporting real division. Minimal complete definition: `fromRational` and (`recip` or `(`/`)`)
Methods

(/) :: a -> a -> a | Source

fractional division

recip :: a -> a | Source

reciprocal fraction

fromRational :: Rational -> a | Source

Conversion from a `Rational` (that is ``Ratio` `Integer``). A floating literal stands for an application of `fromRational` to a value of type `Rational`, so such literals have type `(`Fractional` a) => a`.

Instances

Fractional Double
Fractional Float
Fractional CLDouble
Fractional CDouble
Fractional CFloat
Integral a => Fractional (Ratio a)
RealFloat a => Fractional (Complex a)
HasResolution a => Fractional (Fixed a)

class Fractional a => Floating a where | Source

Trigonometric and hyperbolic functions and related functions. Minimal complete definition: `pi`, `exp`, `log`, `sin`, `cos`, `sinh`, `cosh`, `asin`, `acos`, `atan`, `asinh`, `acosh` and `atanh`
Methods

pi :: a | Source

exp :: a -> a | Source

sqrt :: a -> a | Source

log :: a -> a | Source

(**) :: a -> a -> a | Source

logBase :: a -> a -> a | Source

sin :: a -> a | Source

tan :: a -> a | Source

cos :: a -> a | Source

asin :: a -> a | Source

atan :: a -> a | Source

acos :: a -> a | Source

sinh :: a -> a | Source

tanh :: a -> a | Source

cosh :: a -> a | Source

asinh :: a -> a | Source

atanh :: a -> a | Source

acosh :: a -> a | Source

Instances

Floating Double
Floating Float
Floating CLDouble
Floating CDouble
Floating CFloat
RealFloat a => Floating (Complex a)

class (Real a, Fractional a) => RealFrac a where | Source

Extracting components of fractions. Minimal complete definition: `properFraction`
Methods

properFraction :: Integral b => a -> (b, a) | Source

The function `properFraction` takes a real fractional number `x` and returns a pair `(n,f)` such that `x = n+f`, and:

- `n` is an integral number with the same sign as `x`; and
- `f` is a fraction with the same type and sign as `x`, and with absolute value less than `1`.

The default definitions of the `ceiling`, `floor`, `truncate` and `round` functions are in terms of `properFraction`.

truncate :: Integral b => a -> b | Source

``truncate` x` returns the integer nearest `x` between zero and `x`

round :: Integral b => a -> b | Source

``round` x` returns the nearest integer to `x`; the even integer if `x` is equidistant between two integers

ceiling :: Integral b => a -> b | Source

``ceiling` x` returns the least integer not less than `x`

floor :: Integral b => a -> b | Source

``floor` x` returns the greatest integer not greater than `x`

Instances

RealFrac Double
RealFrac Float
RealFrac CLDouble
RealFrac CDouble
RealFrac CFloat
Integral a => RealFrac (Ratio a)
HasResolution a => RealFrac (Fixed a)

class (RealFrac a, Floating a) => RealFloat a where | Source

Efficient, machine-independent access to the components of a floating-point number. Minimal complete definition: all except `exponent`, `significand`, `scaleFloat` and `atan2`
Methods

floatRadix :: a -> Integer | Source

a constant function, returning the radix of the representation (often `2`)

floatDigits :: a -> Int | Source

a constant function, returning the number of digits of `floatRadix` in the significand

floatRange :: a -> (Int, Int) | Source

a constant function, returning the lowest and highest values the exponent may assume

decodeFloat :: a -> (Integer, Int) | Source

The function `decodeFloat` applied to a real floating-point number returns the significand expressed as an `Integer` and an appropriately scaled exponent (an `Int`). If ``decodeFloat` x` yields `(m,n)`, then `x` is equal in value to `m*b^^n`, where `b` is the floating-point radix, and furthermore, either `m` and `n` are both zero or else `b^(d-1) <= m < b^d`, where `d` is the value of ``floatDigits` x`. In particular, ``decodeFloat` 0 = (0,0)`.

encodeFloat :: Integer -> Int -> a | Source

`encodeFloat` performs the inverse of `decodeFloat`

exponent :: a -> Int | Source

the second component of `decodeFloat`.

significand :: a -> a | Source

the first component of `decodeFloat`, scaled to lie in the open interval (`-1`,`1`)

scaleFloat :: Int -> a -> a | Source

multiplies a floating-point number by an integer power of the radix

isNaN :: a -> Bool | Source

`True` if the argument is an IEEE "not-a-number" (NaN) value

isInfinite :: a -> Bool | Source

`True` if the argument is an IEEE infinity or negative infinity

isDenormalized :: a -> Bool | Source

`True` if the argument is too small to be represented in normalized format

isNegativeZero :: a -> Bool | Source

`True` if the argument is an IEEE negative zero

isIEEE :: a -> Bool | Source

`True` if the argument is an IEEE floating point number

atan2 :: a -> a -> a | Source

a version of arctangent taking two real floating-point arguments. For real floating `x` and `y`, ``atan2` y x` computes the angle (from the positive x-axis) of the vector from the origin to the point `(x,y)`. ``atan2` y x` returns a value in the range [`-pi`, `pi`]. It follows the Common Lisp semantics for the origin when signed zeroes are supported. ``atan2` y 1`, with `y` in a type that is `RealFloat`, should return the same value as ``atan` y`. A default definition of `atan2` is provided, but implementors can provide a more accurate implementation.

Instances

RealFloat Double
RealFloat Float
RealFloat CLDouble
RealFloat CDouble
RealFloat CFloat

Numeric functions

subtract :: Num a => a -> a -> a | Source

the same as ``flip` (`-`)`. Because `-` is treated specially in the Haskell grammar, `(-` e`)` is not a section, but an application of prefix negation. However, `(`subtract`` exp`)` is equivalent to the disallowed section.

even :: Integral a => a -> Bool | Source

odd :: Integral a => a -> Bool | Source

gcd :: Integral a => a -> a -> a | Source

``gcd` x y` is the greatest (positive) integer that divides both `x` and `y`; for example ``gcd` (-3) 6` = `3`, ``gcd` (-3) (-6)` = `3`, ``gcd` 0 4` = `4`. ``gcd` 0 0` raises a runtime error.

lcm :: Integral a => a -> a -> a | Source

``lcm` x y` is the smallest positive integer that both `x` and `y` divide.

(^) :: (Num a, Integral b) => a -> b -> a | Source

raise a number to a non-negative integral power

(^^) :: (Fractional a, Integral b) => a -> b -> a | Source

raise a number to an integral power

fromIntegral :: (Integral a, Num b) => a -> b | Source

general coercion from integral types

realToFrac :: (Real a, Fractional b) => a -> b | Source

general coercion to fractional types
Monads and functors

class Monad m where | Source

The `Monad` class defines the basic operations over a monad, a concept from a branch of mathematics known as category theory. From the perspective of a Haskell programmer, however, it is best to think of a monad as an abstract datatype of actions. Haskell's `do` expressions provide a convenient syntax for writing monadic expressions.

Minimal complete definition: `>>=` and `return`.

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, Data.Maybe.Maybe and System.IO.IO defined in the Prelude satisfy these laws.

Methods

(>>=) :: forall a b. m a -> (a -> m b) -> m b | Source

Sequentially compose two actions, passing any value produced by the first as an argument to the second.

(>>) :: forall a b. m a -> m b -> m b | Source

Sequentially compose two actions, discarding any value produced by the first, like sequencing operators (such as the semicolon) in imperative languages.

return :: a -> m a | Source

Inject a value into the monadic type.

fail :: String -> m a | Source

Fail with a message. This operation is not part of the mathematical definition of a monad, but is invoked on pattern-match failure in a `do` expression.

Instances

Monad []
Monad IO
Monad [::]
Monad Maybe
Monad ReadP
Monad P
Monad ReadPrec
Monad STM
Monad ((->) r)
Monad (ST s)
ArrowApply a => Monad (ArrowMonad a)
Monad (ST s)

class Functor f where | Source

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, Data.Maybe.Maybe and System.IO.IO defined in the Prelude satisfy these laws.

Methods

fmap :: (a -> b) -> f a -> f b | Source

Instances

Functor []
Functor IO
Functor [::]
Functor Maybe
Functor ReadP
Functor ReadPrec
Functor STM
Functor ZipList
Functor Id
Functor ((->) r)
Functor (Either a)
Functor ((,) a)
Functor (ST s)
Ix i => Functor (Array i)
Functor (ST s)
Monad m => Functor (WrappedMonad m)
Functor (Const m)
Functor (StateR s)
Functor (StateL s)
Arrow a => Functor (WrappedArrow a b)

mapM :: Monad m => (a -> m b) -> [a] -> m [b] | Source

``mapM` f` is equivalent to ``sequence` . `map` f`.

mapM_ :: Monad m => (a -> m b) -> [a] -> m () | Source

``mapM_` f` is equivalent to ``sequence_` . `map` f`.

sequence :: Monad m => [m a] -> m [a] | Source

Evaluate each action in the sequence from left to right, and collect the results.

sequence_ :: Monad m => [m a] -> m () | Source

Evaluate each action in the sequence from left to right, and ignore the results.

(=<<) :: Monad m => (a -> m b) -> m a -> m b | Source

Same as `>>=`, but with the arguments interchanged.
Miscellaneous functions

id :: a -> a | Source

Identity function.

const :: a -> b -> a | Source

Constant function.

(.) :: (b -> c) -> (a -> b) -> a -> c | Source

Function composition.

flip :: (a -> b -> c) -> b -> a -> c | Source

``flip` f` takes its (first) two arguments in the reverse order of `f`.

($) :: (a -> b) -> a -> b | Source

Application operator. This operator is redundant, since ordinary application `(f x)` means the same as `(f `$` x)`. However, `$` has low, right-associative binding precedence, so it sometimes allows parentheses to be omitted; for example:

         f $ g $ h x  =  f (g (h x))

It is also useful in higher-order situations, such as ``map` (`$` 0) xs`, or `Data.List.zipWith (`$`) fs xs`.

until :: (a -> Bool) -> (a -> a) -> a -> a | Source

``until` p f` yields the result of applying `f` until `p` holds.

asTypeOf :: a -> a -> a | Source

`asTypeOf` is a type-restricted version of `const`. It is usually used as an infix operator, and its typing forces its first argument (which is usually overloaded) to have the same type as the second.

error :: [Char] -> a | Source

`error` stops execution and displays an error message.

undefined :: a | Source

A special case of `error`. It is expected that compilers will recognize this and insert error messages which are more appropriate to the context in which `undefined` appears.
seq

($!) :: (a -> b) -> a -> b | Source

Strict (call-by-value) application, defined in terms of `seq`.
List operations

map :: (a -> b) -> [a] -> [b] | Source

`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] | Source

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] | Source

`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 | Source

Extract the first element of a list, which must be non-empty.

last :: [a] -> a | Source

Extract the last element of a list, which must be finite and non-empty.

tail :: [a] -> [a] | Source

Extract the elements after the head of a list, which must be non-empty.

init :: [a] -> [a] | Source

Return all the elements of a list except the last one. The list must be finite and non-empty.

null :: [a] -> Bool | Source

Test whether a list is empty.

length :: [a] -> Int | Source

`length` returns the length of a finite list as an `Int`. It is an instance of the more general Data.List.genericLength, the result type of which may be any kind of number.

(!!) :: [a] -> Int -> a | Source

List index (subscript) operator, starting from 0. It is an instance of the more general Data.List.genericIndex, which takes an index of any integral type.

reverse :: [a] -> [a] | Source

`reverse` `xs` returns the elements of `xs` in reverse order. `xs` must be finite.
Reducing lists (folds)

foldl :: (a -> b -> a) -> a -> [b] -> a | Source

`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 | Source

`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 | Source

`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 | Source

`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 | Source

`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 | Source

`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 | Source

Applied to a predicate and a list, `any` determines if any element of the list satisfies the predicate.

all :: (a -> Bool) -> [a] -> Bool | Source

Applied to a predicate and a list, `all` determines if all elements of the list satisfy the predicate.

sum :: Num a => [a] -> a | Source

The `sum` function computes the sum of a finite list of numbers.

product :: Num a => [a] -> a | Source

The `product` function computes the product of a finite list of numbers.

concat :: [[a]] -> [a] | Source

Concatenate a list of lists.

concatMap :: (a -> [b]) -> [a] -> [b] | Source

Map a function over a list and concatenate the results.

maximum :: Ord a => [a] -> a | Source

`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 | Source

`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] | Source

`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] | Source

`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] | Source

`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] | Source

`scanr1` is a variant of `scanr` that has no starting value argument.
Infinite lists

iterate :: (a -> a) -> a -> [a] | Source

`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] | Source

`repeat` `x` is an infinite list, with `x` the value of every element.

replicate :: Int -> a -> [a] | Source

`replicate` `n x` is a list of length `n` with `x` the value of every element. It is an instance of the more general Data.List.genericReplicate, in which `n` may be of any integral type.

cycle :: [a] -> [a] | Source

`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] | Source

`take` `n`, applied to a list `xs`, returns the prefix of `xs` of length `n`, or `xs` itself if `n > `length` xs`:

     take 5 "Hello World!" == "Hello"
     take 3 [1,2,3,4,5] == [1,2,3]
     take 3 [1,2] == [1,2]
     take 3 [] == []
     take (-1) [1,2] == []
     take 0 [1,2] == []

It is an instance of the more general Data.List.genericTake, in which `n` may be of any integral type.

drop :: Int -> [a] -> [a] | Source

`drop` `n xs` returns the suffix of `xs` after the first `n` elements, or `[]` if `n > `length` xs`:

     drop 6 "Hello World!" == "World!"
     drop 3 [1,2,3,4,5] == [4,5]
     drop 3 [1,2] == []
     drop 3 [] == []
     drop (-1) [1,2] == [1,2]
     drop 0 [1,2] == [1,2]

It is an instance of the more general Data.List.genericDrop, in which `n` may be of any integral type.

splitAt :: Int -> [a] -> ([a], [a]) | Source

`splitAt` `n xs` returns a tuple where first element is `xs` prefix of length `n` and second element is the remainder of the list:

     splitAt 6 "Hello World!" == ("Hello ","World!")
     splitAt 3 [1,2,3,4,5] == ([1,2,3],[4,5])
     splitAt 1 [1,2,3] == ([1],[2,3])
     splitAt 3 [1,2,3] == ([1,2,3],[])
     splitAt 4 [1,2,3] == ([1,2,3],[])
     splitAt 0 [1,2,3] == ([],[1,2,3])
     splitAt (-1) [1,2,3] == ([],[1,2,3])

It is equivalent to `(`take` n xs, `drop` n xs)`. `splitAt` is an instance of the more general Data.List.genericSplitAt, in which `n` may be of any integral type.

takeWhile :: (a -> Bool) -> [a] -> [a] | Source

`takeWhile`, applied to a predicate `p` and a list `xs`, returns the longest prefix (possibly empty) of `xs` of elements that satisfy `p`:

     takeWhile (< 3) [1,2,3,4,1,2,3,4] == [1,2]
     takeWhile (< 9) [1,2,3] == [1,2,3]
     takeWhile (< 0) [1,2,3] == []

dropWhile :: (a -> Bool) -> [a] -> [a] | Source

`dropWhile` `p xs` returns the suffix remaining after `takeWhile` `p xs`:

     dropWhile (< 3) [1,2,3,4,5,1,2,3] == [3,4,5,1,2,3]
     dropWhile (< 9) [1,2,3] == []
     dropWhile (< 0) [1,2,3] == [1,2,3]

span :: (a -> Bool) -> [a] -> ([a], [a]) | Source

`span`, applied to a predicate `p` and a list `xs`, returns a tuple where first element is longest prefix (possibly empty) of `xs` of elements that satisfy `p` and second element is the remainder of the list:

     span (< 3) [1,2,3,4,1,2,3,4] == ([1,2],[3,4,1,2,3,4])
     span (< 9) [1,2,3] == ([1,2,3],[])
     span (< 0) [1,2,3] == ([],[1,2,3])

`span` `p xs` is equivalent to `(`takeWhile` p xs, `dropWhile` p xs)`

break :: (a -> Bool) -> [a] -> ([a], [a]) | Source

`break`, applied to a predicate `p` and a list `xs`, returns a tuple where first element is longest prefix (possibly empty) of `xs` of elements that do not satisfy `p` and second element is the remainder of the list:

     break (> 3) [1,2,3,4,1,2,3,4] == ([1,2,3],[4,1,2,3,4])
     break (< 9) [1,2,3] == ([],[1,2,3])
     break (> 9) [1,2,3] == ([1,2,3],[])

`break` `p` is equivalent to ``span` (`not` . p)`.

Searching lists

elem :: Eq a => a -> [a] -> Bool | Source

`elem` is the list membership predicate, usually written in infix form, e.g., `x `elem` xs`.

notElem :: Eq a => a -> [a] -> Bool | Source

`notElem` is the negation of `elem`.

lookup :: Eq a => a -> [(a, b)] -> Maybe b | Source

`lookup` `key assocs` looks up a key in an association list.
Zipping and unzipping lists

zip :: [a] -> [b] -> [(a, b)] | Source

`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)] | Source

`zip3` takes three lists and returns a list of triples, analogous to `zip`.

zipWith :: (a -> b -> c) -> [a] -> [b] -> [c] | Source

`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] | Source

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]) | Source

`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]) | Source

The `unzip3` function takes a list of triples and returns three lists, analogous to `unzip`.
Functions on strings

lines :: String -> [String] | Source

`lines` breaks a string up into a list of strings at newline characters. The resulting strings do not contain newlines.

words :: String -> [String] | Source

`words` breaks a string up into a list of words, which were delimited by white space.

unlines :: [String] -> String | Source

`unlines` is an inverse operation to `lines`. It joins lines, after appending a terminating newline to each.

unwords :: [String] -> String | Source

`unwords` is an inverse operation to `words`. It joins words with separating spaces.
Converting to and from `String`
Converting to `String`

type ShowS = String -> String | Source

The `shows` functions return a function that prepends the output `String` to an existing `String`. This allows constant-time concatenation of results using function composition.

class Show a where | Source

Conversion of values to readable `String`s.

Minimal complete definition: `showsPrec` or `show`.

Derived instances of `Show` have the following properties, which are compatible with derived instances of Text.Read.Read:

- The result of `show` is a syntactically correct Haskell expression containing only constants, given the fixity declarations in force at the point where the type is declared. It contains only the constructor names defined in the data type, parentheses, and spaces. When labelled constructor fields are used, braces, commas, field names, and equal signs are also used.
- If the constructor is defined to be an infix operator, then `showsPrec` will produce infix applications of the constructor.
- the representation will be enclosed in parentheses if the precedence of the top-level constructor in `x` is less than `d` (associativity is ignored). Thus, if `d` is `0` then the result is never surrounded in parentheses; if `d` is `11` it is always surrounded in parentheses, unless it is an atomic expression.
- If the constructor is defined using record syntax, then `show` will produce the record-syntax form, with the fields given in the same order as the original declaration.

For example, given the declarations

     infixr 5 :^:
     data Tree a =  Leaf a  |  Tree a :^: Tree a

the derived instance of `Show` is equivalent to

     instance (Show a) => Show (Tree a) where

            showsPrec d (Leaf m) = showParen (d > app_prec) $
                 showString "Leaf " . showsPrec (app_prec+1) m
              where app_prec = 10

            showsPrec d (u :^: v) = showParen (d > up_prec) $
                 showsPrec (up_prec+1) u .
                 showString " :^: "      .
                 showsPrec (up_prec+1) v
              where up_prec = 5

Note that right-associativity of `:^:` is ignored. For example,

- ``show` (Leaf 1 :^: Leaf 2 :^: Leaf 3)` produces the string `"Leaf 1 :^: (Leaf 2 :^: Leaf 3)"`.

Methods

showsPrec | Source

:: Int | the operator precedence of the enclosing context (a number from `0` to `11`). Function application has precedence `10`.
-> a | the value to be converted to a `String`
-> ShowS |

Convert a value to a readable `String`.

`showsPrec` should satisfy the law

     showsPrec d x r ++ s  ==  showsPrec d x (r ++ s)

Derived instances of Text.Read.Read and `Show` satisfy the following:

- `(x,"")` is an element of `(Text.Read.readsPrec d (`showsPrec` d x ""))`.

That is, Text.Read.readsPrec parses the string produced by `showsPrec`, and delivers the value that `showsPrec` started with.

show :: a -> String | Source

A specialised variant of `showsPrec`, using precedence context zero, and returning an ordinary `String`.

showList :: [a] -> ShowS | Source

The method `showList` is provided to allow the programmer to give a specialised way of showing lists of values. For example, this is used by the predefined `Show` instance of the `Char` type, where values of type `String` should be shown in double quotes, rather than between square brackets.

Instances

Show Bool
Show Char
Show Double
Show Float
Show Int
Show Int8
Show Int16
Show Int32
Show Int64
Show Integer
Show Ordering
Show Word
Show Word8
Show Word16
Show Word32
Show Word64
Show ()
Show TyCon
Show TypeRep
Show ArithException
Show ErrorCall
Show SomeException
Show Lexeme
Show CUIntMax
Show CIntMax
Show CUIntPtr
Show CIntPtr
Show CTime
Show CClock
Show CSigAtomic
Show CWchar
Show CSize
Show CPtrdiff
Show CLDouble
Show CDouble
Show CFloat
Show CULLong
Show CLLong
Show CULong
Show CLong
Show CUInt
Show CInt
Show CUShort
Show CShort
Show CUChar
Show CSChar
Show CChar
Show IOMode
Show IOErrorType
Show IOException
Show ExitCode
Show ArrayException
Show AsyncException
Show AssertionFailed
Show Deadlock
Show BlockedIndefinitely
Show BlockedOnDeadMVar
Show BufferMode
Show HandleType
Show Handle
Show GeneralCategory
Show HashData
Show Dynamic
Show IntPtr
Show WordPtr
Show Fd
Show CRLim
Show CTcflag
Show CSpeed
Show CCc
Show CUid
Show CNlink
Show CGid
Show CSsize
Show CPid
Show COff
Show CMode
Show CIno
Show CDev
Show ThreadStatus
Show BlockReason
Show ThreadId
Show NestedAtomically
Show NonTermination
Show NoMethodError
Show RecUpdError
Show RecConError
Show RecSelError
Show PatternMatchFail
Show SeekMode
Show HandlePosn
Show Exception
Show Fixity
Show ConstrRep
Show DataRep
Show Constr
Show DataType
Show Any
Show All
Show Timeout
Show Version
Show a => Show ([] a)
Integral a => Show (Ratio a)
Show (Ptr a)
Show (FunPtr a)
Show a => Show ([::] a)
Show a => Show (Maybe a)
Show (ForeignPtr a)
RealFloat a => Show (Complex a)
HasResolution a => Show (Fixed a)
Show a => Show (Last a)
Show a => Show (First a)
Show a => Show (Product a)
Show a => Show (Sum a)
Show a => Show (Dual a)
Show (a -> b)
(Show a, Show b) => Show (Either a b)
(Show a, Show b) => Show ((,) a b)
Show (ST s a)
(Ix a, Show a, Show b) => Show (Array 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 a, Show b, Show c, Show d, Show e, Show f) => Show ((,,,,,) a b c d e f)
(Show a, Show b, Show c, Show d, Show e, Show f, Show g) => Show ((,,,,,,) a b c d e f g)
(Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h) => Show ((,,,,,,,) a b c d e f g h)
(Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i) => Show ((,,,,,,,,) a b c d e f g h i)
(Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i, Show j) => Show ((,,,,,,,,,) a b c d e f g h i j)
(Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i, Show j, Show k) => Show ((,,,,,,,,,,) a b c d e f g h i j k)
(Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i, Show j, Show k, Show l) => Show ((,,,,,,,,,,,) a b c d e f g h i j k l)
(Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i, Show j, Show k, Show l, Show m) => Show ((,,,,,,,,,,,,) a b c d e f g h i j k l m)
(Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i, Show j, Show k, Show l, Show m, Show n) => Show ((,,,,,,,,,,,,,) a b c d e f g h i j k l m n)
(Show a, Show b, Show c, Show d, Show e, Show f, Show g, Show h, Show i, Show j, Show k, Show l, Show m, Show n, Show o) => Show ((,,,,,,,,,,,,,,) a b c d e f g h i j k l m n o)

shows :: Show a => a -> ShowS | Source

equivalent to `showsPrec` with a precedence of 0.

showChar :: Char -> ShowS | Source

utility function converting a `Char` to a show function that simply prepends the character unchanged.

showString :: String -> ShowS | Source

utility function converting a `String` to a show function that simply prepends the string unchanged.

showParen :: Bool -> ShowS -> ShowS | Source

utility function that surrounds the inner show function with parentheses when the `Bool` parameter is `True`.
Converting from `String`

type ReadS a = String -> [(a, String)] | Source

A parser for a type `a`, represented as a function that takes a `String` and returns a list of possible parses as `(a,`String`)` pairs. Note that this kind of backtracking parser is very inefficient; reading a large structure may be quite slow (cf `ReadP`).

class Read a where | Source

Parsing of `String`s, producing values.

Minimal complete definition: `readsPrec` (or, for GHC only, `readPrec`)

Derived instances of `Read` make the following assumptions, which derived instances of Text.Show.Show obey:

- If the constructor is defined to be an infix operator, then the derived `Read` instance will parse only infix applications of the constructor (not the prefix form).
- Associativity is not used to reduce the occurrence of parentheses, although precedence may be.
- If the constructor is defined using record syntax, the derived `Read` will parse only the record-syntax form, and furthermore, the fields must be given in the same order as the original declaration.
- The derived `Read` instance allows arbitrary Haskell whitespace between tokens of the input string. Extra parentheses are also allowed.

For example, given the declarations

     infixr 5 :^:
     data Tree a =  Leaf a  |  Tree a :^: Tree a

the derived instance of `Read` in Haskell 98 is equivalent to

     instance (Read a) => Read (Tree a) where

             readsPrec d r =  readParen (d > app_prec)
                              (\r -> [(Leaf m,t) |
                                      ("Leaf",s) <- lex r,
                                      (m,t) <- readsPrec (app_prec+1) s]) r

                           ++ readParen (d > up_prec)
                              (\r -> [(u:^:v,w) |
                                      (u,s) <- readsPrec (up_prec+1) r,
                                      (":^:",t) <- lex s,
                                      (v,w) <- readsPrec (up_prec+1) t]) r

               where app_prec = 10
                     up_prec = 5

Note that right-associativity of `:^:` is unused.

The derived instance in GHC is equivalent to

     instance (Read a) => Read (Tree a) where

             readPrec = parens $ (prec app_prec $ do
                                      Ident "Leaf" <- lexP
                                      m <- step readPrec
                                      return (Leaf m))

                          +++ (prec up_prec $ do
                                      u <- step readPrec
                                      Symbol ":^:" <- lexP
                                      v <- step readPrec
                                      return (u :^: v))

               where app_prec = 10
                     up_prec = 5

             readListPrec = readListPrecDefault

Methods

readsPrec | Source

:: Int | the operator precedence of the enclosing context (a number from `0` to `11`). Function application has precedence `10`.
-> ReadS a |

attempts to parse a value from the front of the string, returning a list of (parsed value, remaining string) pairs. If there is no successful parse, the returned list is empty.

Derived instances of `Read` and Text.Show.Show satisfy the following:

- `(x,"")` is an element of `(`readsPrec` d (Text.Show.showsPrec d x ""))`.

That is, `readsPrec` parses the string produced by Text.Show.showsPrec, and delivers the value that Text.Show.showsPrec started with.

readList :: ReadS [a] | Source

The method `readList` is provided to allow the programmer to give a specialised way of parsing lists of values. For example, this is used by the predefined `Read` instance of the `Char` type, where values of type `String` should be are expected to use double quotes, rather than square brackets.

Instances

Read Bool
Read Char
Read Double
Read Float
Read Int
Read Int8
Read Int16
Read Int32
Read Int64
Read Integer
Read Ordering
Read Word
Read Word8
Read Word16
Read Word32
Read Word64
Read ()
Read Lexeme
Read CUIntMax
Read CIntMax
Read CUIntPtr
Read CIntPtr
Read CTime
Read CClock
Read CSigAtomic
Read CWchar
Read CSize
Read CPtrdiff
Read CLDouble
Read CDouble
Read CFloat
Read CULLong
Read CLLong
Read CULong
Read CLong
Read CUInt
Read CInt
Read CUShort
Read CShort
Read CUChar
Read CSChar
Read CChar
Read IOMode
Read ExitCode
Read BufferMode
Read GeneralCategory
Read IntPtr
Read WordPtr
Read Fd
Read CRLim
Read CTcflag
Read CSpeed
Read CCc
Read CUid
Read CNlink
Read CGid
Read CSsize
Read CPid
Read COff
Read CMode
Read CIno
Read CDev
Read SeekMode
Read Any
Read All
Read Version
Read a => Read ([] a)
(Integral a, Read a) => Read (Ratio a)
Read a => Read ([::] a)
Read a => Read (Maybe a)
(Read a, RealFloat a) => Read (Complex a)
Read a => Read (Last a)
Read a => Read (First a)
Read a => Read (Product a)
Read a => Read (Sum a)
Read a => Read (Dual a)
(Read a, Read b) => Read (Either a b)
(Read a, Read b) => Read ((,) a b)
(Ix a, Read a, Read b) => Read (Array 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 a, Read b, Read c, Read d, Read e, Read f) => Read ((,,,,,) a b c d e f)
(Read a, Read b, Read c, Read d, Read e, Read f, Read g) => Read ((,,,,,,) a b c d e f g)
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h) => Read ((,,,,,,,) a b c d e f g h)
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i) => Read ((,,,,,,,,) a b c d e f g h i)
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j) => Read ((,,,,,,,,,) a b c d e f g h i j)
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k) => Read ((,,,,,,,,,,) a b c d e f g h i j k)
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k, Read l) => Read ((,,,,,,,,,,,) a b c d e f g h i j k l)
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k, Read l, Read m) => Read ((,,,,,,,,,,,,) a b c d e f g h i j k l m)
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k, Read l, Read m, Read n) => Read ((,,,,,,,,,,,,,) a b c d e f g h i j k l m n)
(Read a, Read b, Read c, Read d, Read e, Read f, Read g, Read h, Read i, Read j, Read k, Read l, Read m, Read n, Read o) => Read ((,,,,,,,,,,,,,,) a b c d e f g h i j k l m n o)

reads :: Read a => ReadS a | Source

equivalent to `readsPrec` with a precedence of 0.

readParen :: Bool -> ReadS a -> ReadS a | Source

``readParen` `True` p` parses what `p` parses, but surrounded with parentheses. ``readParen` `False` p` parses what `p` parses, but optionally surrounded with parentheses.

read :: Read a => String -> a | Source

The `read` function reads input from a string, which must be completely consumed by the input process.

lex :: ReadS String | Source

The `lex` function reads a single lexeme from the input, discarding initial white space, and returning the characters that constitute the lexeme. If the input string contains only white space, `lex` returns a single successful `lexeme' consisting of the empty string. (Thus ``lex` "" = [("","")]`.) If there is no legal lexeme at the beginning of the input string, `lex` fails (i.e. returns `[]`).

This lexer is not completely faithful to the Haskell lexical syntax in the following respects:

- Qualified names are not handled properly
- Octal and hexadecimal numerics are not recognized as a single token
- Comments are not treated properly

Basic Input and output

data IO a | Source

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

Monad IO
Functor IO
Typeable1 IO
MonadFix IO
Applicative IO
HPrintfType (IO a)
PrintfType (IO a)

Simple I/O operations
Output functions

putChar :: Char -> IO () | Source

Write a character to the standard output device (same as `hPutChar` `stdout`).

putStr :: String -> IO () | Source

Write a string to the standard output device (same as `hPutStr` `stdout`).

putStrLn :: String -> IO () | Source

The same as `putStr`, but adds a newline character.

print :: Show a => a -> IO () | Source

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 | Source

Read a character from the standard input device (same as `hGetChar` `stdin`).

getLine :: IO String | Source

Read a line from the standard input device (same as `hGetLine` `stdin`).

getContents :: IO String | Source

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 () | Source

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 | Source

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 | Source

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 () | Source

The computation `writeFile` `file str` function writes the string `str`, to the file `file`.

appendFile :: FilePath -> String -> IO () | Source

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 | Source

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 | Source

The `readLn` function combines `getLine` and `readIO`.
Exception handling in the I/O monad

type IOError = IOException | Source

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 Control.Exception.Exception. In Haskell 98, this is an opaque type.

ioError :: IOError -> IO a | Source

Raise an `IOError` in the `IO` monad.

userError :: String -> IOError | Source

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 | Source

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 `IOError`s 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. System.IO.Error.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 Control.Exception.catch from Control.Exception.

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