In this appendix the entire Haskell Prelude is given. It constitutes a specification for the Prelude. Many of the definitions are written with clarity rather than efficiency in mind, and it is not required that the specification be implemented as shown here.
The default method definitions, given with class declarations, constitute a specification only of the default method. They do not constitute a specification of the meaning of the method in all instances. To take one particular example, the default method for enumFrom in class Enum will not work properly for types whose range exceeds that of Int (because fromEnum cannot map all values in the type to distinct Int values).
The Prelude shown here is organized into a root module, Prelude, and three sub-modules, PreludeList, PreludeText, and PreludeIO. This structure is purely presentational. An implementation is not required to use this organisation for the Prelude, nor are these three modules available for import separately. Only the exports of module Prelude are significant.
Some of these modules import Library modules, such as Char, Monad, IO, and Numeric. These modules are described fully in the accompanying Haskell 98 Library Report. These imports are not, of course, part of the specification of the Prelude. That is, an implementation is free to import more, or less, of the Library modules, as it pleases.
Primitives that are not definable in Haskell , indicated by names starting with "prim", are defined in a system dependent manner in module PreludeBuiltin and are not shown here. Instance declarations that simply bind primitives to class methods are omitted. Some of the more verbose instances with obvious functionality have been left out for the sake of brevity.
Declarations for special types such as Integer, or () are included in the Prelude for completeness even though the declaration may be incomplete or syntactically invalid. An ellipsis "..." is often used in places where the remainder of a definition cannot be given in Haskell.
To reduce the occurrence of unexpected ambiguity errors, and to improve efficiency, a number of commonly-used functions over lists use the Int type rather than using a more general numeric type, such as Integral a or Num a. These functions are: take, drop, !!, length, splitAt, and replicate. The more general versions are given in the List library, with the prefix "generic"; for example genericLength.
module Prelude (
module PreludeList, module PreludeText, module PreludeIO,
Bool(False, True),
Maybe(Nothing, Just),
Either(Left, Right),
Ordering(LT, EQ, GT),
Char, String, Int, Integer, Float, Double, Rational, IO,
-- These built-in types are defined in the Prelude, but
-- are denoted by built-in syntax, and cannot legally
-- appear in an export list.
-- List type: []((:), [])
-- Tuple types: (,)((,)), (,,)((,,)), etc.
-- Trivial type: ()(())
-- Functions: (->)
Eq((==), (/=)),
Ord(compare, (<), (<=), (>=), (>), max, min),
Enum(succ, pred, toEnum, fromEnum, enumFrom, enumFromThen,
enumFromTo, enumFromThenTo),
Bounded(minBound, maxBound),
Num((+), (-), (*), negate, abs, signum, fromInteger),
Real(toRational),
Integral(quot, rem, div, mod, quotRem, divMod, toInteger),
Fractional((/), recip, fromRational),
Floating(pi, exp, log, sqrt, (**), logBase, sin, cos, tan,
asin, acos, atan, sinh, cosh, tanh, asinh, acosh, atanh),
RealFrac(properFraction, truncate, round, ceiling, floor),
RealFloat(floatRadix, floatDigits, floatRange, decodeFloat,
encodeFloat, exponent, significand, scaleFloat, isNaN,
isInfinite, isDenormalized, isIEEE, isNegativeZero, atan2),
Monad((>>=), (>>), return, fail),
Functor(fmap),
mapM, mapM_, sequence, sequence_, (=<<),
maybe, either,
(&&), (||), not, otherwise,
subtract, even, odd, gcd, lcm, (^), (^^),
fromIntegral, realToFrac,
fst, snd, curry, uncurry, id, const, (.), flip, ($), until,
asTypeOf, error, undefined,
seq, ($!)
) where
import PreludeBuiltin -- Contains all `prim' values
import UnicodePrims( primUnicodeMaxChar ) -- Unicode primitives
import PreludeList
import PreludeText
import PreludeIO
import Ratio( Rational )
infixr 9 .
infixr 8 ^, ^^, **
infixl 7 *, /, `quot`, `rem`, `div`, `mod`
infixl 6 +, -
-- The (:) operator is built-in syntax, and cannot legally be given
-- a fixity declaration; but its fixity is given by:
-- infixr 5 :
infix 4 ==, /=, <, <=, >=, >
infixr 3 &&
infixr 2 ||
infixl 1 >>, >>=
infixr 1 =<<
infixr 0 $, $!, `seq`
-- Standard types, classes, instances and related functions
-- Equality and Ordered classes
class Eq a where
(==), (/=) :: a -> a -> Bool
-- Minimal complete definition:
-- (==) or (/=)
x /= y = not (x == y)
x == y = not (x /= y)
class (Eq a) => Ord a where
compare :: a -> a -> Ordering
(<), (<=), (>=), (>) :: a -> a -> Bool
max, min :: a -> a -> a
-- Minimal complete definition:
-- (<=) or compare
-- Using compare can be more efficient for complex types.
compare x y
| x == y = EQ
| x <= y = LT
| otherwise = GT
x <= y = compare x y /= GT
x < y = compare x y == LT
x >= y = compare x y /= LT
x > y = compare x y == GT
-- note that (min x y, max x y) = (x,y) or (y,x)
max x y
| x <= y = y
| otherwise = x
min x y
| x <= y = x
| otherwise = y
-- Enumeration and Bounded classes
class Enum a where
succ, pred :: a -> a
toEnum :: Int -> a
fromEnum :: a -> Int
enumFrom :: a -> [a] -- [n..]
enumFromThen :: a -> a -> [a] -- [n,n'..]
enumFromTo :: a -> a -> [a] -- [n..m]
enumFromThenTo :: a -> a -> a -> [a] -- [n,n'..m]
-- Minimal complete definition:
-- toEnum, fromEnum
--
-- NOTE: these default methods only make sense for types
-- that map injectively into Int using fromEnum
-- and toEnum.
succ = toEnum . (+1) . fromEnum
pred = toEnum . (subtract 1) . fromEnum
enumFrom x = map toEnum [fromEnum x ..]
enumFromTo x y = map toEnum [fromEnum x .. fromEnum y]
enumFromThen x y = map toEnum [fromEnum x, fromEnum y ..]
enumFromThenTo x y z =
map toEnum [fromEnum x, fromEnum y .. fromEnum z]
class Bounded a where
minBound :: a
maxBound :: a
-- Numeric classes
class (Eq a, Show a) => Num a where
(+), (-), (*) :: a -> a -> a
negate :: a -> a
abs, signum :: a -> a
fromInteger :: Integer -> a
-- Minimal complete definition:
-- All, except negate or (-)
x - y = x + negate y
negate x = 0 - x
class (Num a, Ord a) => Real a where
toRational :: a -> Rational
class (Real a, Enum a) => Integral a where
quot, rem :: a -> a -> a
div, mod :: a -> a -> a
quotRem, divMod :: a -> a -> (a,a)
toInteger :: a -> Integer
-- Minimal complete definition:
-- quotRem, toInteger
n `quot` d = q where (q,r) = quotRem n d
n `rem` d = r where (q,r) = quotRem n d
n `div` d = q where (q,r) = divMod n d
n `mod` d = r where (q,r) = divMod n d
divMod n d = if signum r == - signum d then (q-1, r+d) else qr
where qr@(q,r) = quotRem n d
class (Num a) => Fractional a where
(/) :: a -> a -> a
recip :: a -> a
fromRational :: Rational -> a
-- Minimal complete definition:
-- fromRational and (recip or (/))
recip x = 1 / x
x / y = x * recip y
class (Fractional a) => Floating a where
pi :: a
exp, log, sqrt :: a -> a
(**), logBase :: a -> a -> a
sin, cos, tan :: a -> a
asin, acos, atan :: a -> a
sinh, cosh, tanh :: a -> a
asinh, acosh, atanh :: a -> a
-- Minimal complete definition:
-- pi, exp, log, sin, cos, sinh, cosh
-- asin, acos, atan
-- asinh, acosh, atanh
x ** y = exp (log x * y)
logBase x y = log y / log x
sqrt x = x ** 0.5
tan x = sin x / cos x
tanh x = sinh x / cosh x
class (Real a, Fractional a) => RealFrac a where
properFraction :: (Integral b) => a -> (b,a)
truncate, round :: (Integral b) => a -> b
ceiling, floor :: (Integral b) => a -> b
-- Minimal complete definition:
-- properFraction
truncate x = m where (m,_) = properFraction x
round x = let (n,r) = properFraction x
m = if r < 0 then n - 1 else n + 1
in case signum (abs r - 0.5) of
-1 -> n
0 -> if even n then n else m
1 -> m
ceiling x = if r > 0 then n + 1 else n
where (n,r) = properFraction x
floor x = if r < 0 then n - 1 else n
where (n,r) = properFraction x
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, isInfinite, isDenormalized, isNegativeZero, isIEEE
:: a -> Bool
atan2 :: a -> a -> a
-- Minimal complete definition:
-- All except exponent, significand,
-- scaleFloat, atan2
exponent x = if m == 0 then 0 else n + floatDigits x
where (m,n) = decodeFloat x
significand x = encodeFloat m (- floatDigits x)
where (m,_) = decodeFloat x
scaleFloat k x = encodeFloat m (n+k)
where (m,n) = decodeFloat x
atan2 y x
| x>0 = atan (y/x)
| x==0 && y>0 = pi/2
| x<0 && y>0 = pi + atan (y/x)
|(x<=0 && y<0) ||
(x<0 && isNegativeZero y) ||
(isNegativeZero x && isNegativeZero y)
= -atan2 (-y) x
| y==0 && (x<0 || isNegativeZero x)
= pi -- must be after the previous test on zero y
| x==0 && y==0 = y -- must be after the other double zero tests
| otherwise = x + y -- x or y is a NaN, return a NaN (via +)
-- Numeric functions
subtract :: (Num a) => a -> a -> a
subtract = flip (-)
even, odd :: (Integral a) => a -> Bool
even n = n `rem` 2 == 0
odd = not . even
gcd :: (Integral a) => a -> a -> a
gcd 0 0 = error "Prelude.gcd: gcd 0 0 is undefined"
gcd x y = gcd' (abs x) (abs y)
where gcd' x 0 = x
gcd' x y = gcd' y (x `rem` y)
lcm :: (Integral a) => a -> a -> a
lcm _ 0 = 0
lcm 0 _ = 0
lcm x y = abs ((x `quot` (gcd x y)) * y)
(^) :: (Num a, Integral b) => a -> b -> a
x ^ 0 = 1
x ^ n | n > 0 = f x (n-1) x
where f _ 0 y = y
f x n y = g x n where
g x n | even n = g (x*x) (n `quot` 2)
| otherwise = f x (n-1) (x*y)
_ ^ _ = error "Prelude.^: negative exponent"
(^^) :: (Fractional a, Integral b) => a -> b -> a
x ^^ n = if n >= 0 then x^n else recip (x^(-n))
fromIntegral :: (Integral a, Num b) => a -> b
fromIntegral = fromInteger . toInteger
realToFrac :: (Real a, Fractional b) => a -> b
realToFrac = fromRational . toRational
-- Monadic classes
class Functor f where
fmap :: (a -> b) -> f a -> f 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
-- Minimal complete definition:
-- (>>=), return
m >> k = m >>= \_ -> k
fail s = error s
sequence :: Monad m => [m a] -> m [a]
sequence = foldr mcons (return [])
where mcons p q = p >>= \x -> q >>= \y -> return (x:y)
sequence_ :: Monad m => [m a] -> m ()
sequence_ = foldr (>>) (return ())
-- The xxxM functions take list arguments, but lift the function or
-- list element to a monad type
mapM :: Monad m => (a -> m b) -> [a] -> m [b]
mapM f as = sequence (map f as)
mapM_ :: Monad m => (a -> m b) -> [a] -> m ()
mapM_ f as = sequence_ (map f as)
(=<<) :: Monad m => (a -> m b) -> m a -> m b
f =<< x = x >>= f
-- Trivial type
data () = () deriving (Eq, Ord, Enum, Bounded)
-- Not legal Haskell; for illustration only
-- Function type
-- identity function
id :: a -> a
id x = x
-- constant function
const :: a -> b -> a
const x _ = x
-- function composition
(.) :: (b -> c) -> (a -> b) -> a -> c
f . g = \ x -> f (g x)
-- flip f takes its (first) two arguments in the reverse order of f.
flip :: (a -> b -> c) -> b -> a -> c
flip f x y = f y x
seq :: a -> b -> b
seq = ... -- Primitive
-- right-associating infix application operators
-- (useful in continuation-passing style)
($), ($!) :: (a -> b) -> a -> b
f $ x = f x
f $! x = x `seq` f x
-- Boolean type
data Bool = False | True deriving (Eq, Ord, Enum, Read, Show, Bounded)
-- Boolean functions
(&&), (||) :: Bool -> Bool -> Bool
True && x = x
False && _ = False
True || _ = True
False || x = x
not :: Bool -> Bool
not True = False
not False = True
otherwise :: Bool
otherwise = True
-- Character type
data Char = ... 'a' | 'b' ... -- Unicode values
instance Eq Char where
c == c' = fromEnum c == fromEnum c'
instance Ord Char where
c <= c' = fromEnum c <= fromEnum c'
instance Enum Char where
toEnum = primIntToChar
fromEnum = primCharToInt
enumFrom c = map toEnum [fromEnum c .. fromEnum (maxBound::Char)]
enumFromThen c c' = map toEnum [fromEnum c, fromEnum c' .. fromEnum lastChar]
where lastChar :: Char
lastChar | c' < c = minBound
| otherwise = maxBound
instance Bounded Char where
minBound = '\0'
maxBound = primUnicodeMaxChar
type String = [Char]
-- Maybe type
data Maybe a = Nothing | Just a deriving (Eq, Ord, Read, Show)
maybe :: b -> (a -> b) -> Maybe a -> b
maybe n f Nothing = n
maybe n f (Just x) = f x
instance Functor Maybe where
fmap f Nothing = Nothing
fmap f (Just x) = Just (f x)
instance Monad Maybe where
(Just x) >>= k = k x
Nothing >>= k = Nothing
return = Just
fail s = Nothing
-- Either type
data Either a b = Left a | Right b deriving (Eq, Ord, Read, Show)
either :: (a -> c) -> (b -> c) -> Either a b -> c
either f g (Left x) = f x
either f g (Right y) = g y
-- IO type
data IO a = ... -- abstract
instance Functor IO where
fmap f x = x >>= (return . f)
instance Monad IO where
(>>=) = ...
return = ...
fail s = ioError (userError s)
-- Ordering type
data Ordering = LT | EQ | GT
deriving (Eq, Ord, Enum, Read, Show, Bounded)
-- Standard numeric types. The data declarations for these types cannot
-- be expressed directly in Haskell since the constructor lists would be
-- far too large.
data Int = minBound ... -1 | 0 | 1 ... maxBound
instance Eq Int where ...
instance Ord Int where ...
instance Num Int where ...
instance Real Int where ...
instance Integral Int where ...
instance Enum Int where ...
instance Bounded Int where ...
data Integer = ... -1 | 0 | 1 ...
instance Eq Integer where ...
instance Ord Integer where ...
instance Num Integer where ...
instance Real Integer where ...
instance Integral Integer where ...
instance Enum Integer where ...
data Float
instance Eq Float where ...
instance Ord Float where ...
instance Num Float where ...
instance Real Float where ...
instance Fractional Float where ...
instance Floating Float where ...
instance RealFrac Float where ...
instance RealFloat Float where ...
data Double
instance Eq Double where ...
instance Ord Double where ...
instance Num Double where ...
instance Real Double where ...
instance Fractional Double where ...
instance Floating Double where ...
instance RealFrac Double where ...
instance RealFloat Double where ...
-- The Enum instances for Floats and Doubles are slightly unusual.
-- The `toEnum' function truncates numbers to Int. The definitions
-- of enumFrom and enumFromThen allow floats to be used in arithmetic
-- series: [0,0.1 .. 0.95]. However, roundoff errors make these somewhat
-- dubious. This example may have either 10 or 11 elements, depending on
-- how 0.1 is represented.
instance Enum Float where
succ x = x+1
pred x = x-1
toEnum = fromIntegral
fromEnum = fromInteger . truncate -- may overflow
enumFrom = numericEnumFrom
enumFromThen = numericEnumFromThen
enumFromTo = numericEnumFromTo
enumFromThenTo = numericEnumFromThenTo
instance Enum Double where
succ x = x+1
pred x = x-1
toEnum = fromIntegral
fromEnum = fromInteger . truncate -- may overflow
enumFrom = numericEnumFrom
enumFromThen = numericEnumFromThen
enumFromTo = numericEnumFromTo
enumFromThenTo = numericEnumFromThenTo
numericEnumFrom :: (Fractional a) => a -> [a]
numericEnumFromThen :: (Fractional a) => a -> a -> [a]
numericEnumFromTo :: (Fractional a, Ord a) => a -> a -> [a]
numericEnumFromThenTo :: (Fractional a, Ord a) => a -> a -> a -> [a]
numericEnumFrom = iterate (+1)
numericEnumFromThen n m = iterate (+(m-n)) n
numericEnumFromTo n m = takeWhile (<= m+1/2) (numericEnumFrom n)
numericEnumFromThenTo n n' m = takeWhile p (numericEnumFromThen n n')
where
p | n' > n = (<= m + (n'-n)/2)
| otherwise = (>= m + (n'-n)/2)
-- Lists
data [a] = [] | a : [a] deriving (Eq, Ord)
-- Not legal Haskell; for illustration only
instance Functor [] where
fmap = map
instance Monad [] where
m >>= k = concat (map k m)
return x = [x]
fail s = []
-- Tuples
data (a,b) = (a,b) deriving (Eq, Ord, Bounded)
data (a,b,c) = (a,b,c) deriving (Eq, Ord, Bounded)
-- Not legal Haskell; for illustration only
-- component projections for pairs:
-- (NB: not provided for triples, quadruples, etc.)
fst :: (a,b) -> a
fst (x,y) = x
snd :: (a,b) -> b
snd (x,y) = y
-- curry converts an uncurried function to a curried function;
-- uncurry converts a curried function to a function on pairs.
curry :: ((a, b) -> c) -> a -> b -> c
curry f x y = f (x, y)
uncurry :: (a -> b -> c) -> ((a, b) -> c)
uncurry f p = f (fst p) (snd p)
-- Misc functions
-- until p f yields the result of applying f until p holds.
until :: (a -> Bool) -> (a -> a) -> a -> a
until p f x
| p x = x
| otherwise = until p f (f x)
-- 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.
asTypeOf :: a -> a -> a
asTypeOf = const
-- error stops execution and displays an error message
error :: String -> a
error = primError
-- It is expected that compilers will recognize this and insert error
-- messages that are more appropriate to the context in which undefined
-- appears.
undefined :: a
undefined = error "Prelude.undefined"