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6  Predefined Types and Classes

The Haskell Prelude contains predefined classes, types, and functions that are implicitly imported into every Haskell program. In this chapter, we describe the types and classes found in the Prelude. Most functions are not described in detail here as they can easily be understood from their definitions as given in Chapter 8. Other predefined types such as arrays, complex numbers, and rationals are defined in Part II.

6.1  Standard Haskell Types

These types are defined by the Haskell Prelude. Numeric types are described in Section 6.4. When appropriate, the Haskell definition of the type is given. Some definitions may not be completely valid on syntactic grounds but they faithfully convey the meaning of the underlying type.

6.1.1  Booleans

data  Bool  =  False | True deriving 
                             (Read, Show, Eq, Ord, Enum, Bounded)

The boolean type Bool is an enumeration. The basic boolean functions are && (and), || (or), and not. The name otherwise is defined as True to make guarded expressions more readable.

6.1.2  Characters and Strings

The character type Char is an enumeration whose values represent Unicode characters [11]. The lexical syntax for characters is defined in Section 2.6; character literals are nullary constructors in the datatype Char. Type Char is an instance of the classes Read, Show, Eq, Ord, Enum, and Bounded. The toEnum and fromEnum functions, standard functions from class Enum, map characters to and from the Int type.

Note that ASCII control characters each have several representations in character literals: numeric escapes, ASCII mnemonic escapes, and the \^X notation. In addition, there are the following equivalences: \a and \BEL, \b and \BS, \f and \FF, \r and \CR, \t and \HT, \v and \VT, and \n and \LF.

A string is a list of characters:

type  String  =  [Char]

Strings may be abbreviated using the lexical syntax described in Section 2.6. For example, "A string" abbreviates

[ 'A',' ','s','t','r', 'i','n','g']

6.1.3  Lists

data  [a]  =  [] | a : [a]  deriving (Eq, Ord)

Lists are an algebraic datatype of two constructors, although with special syntax, as described in Section 3.7. The first constructor is the null list, written `[]' ("nil"), and the second is `:' ("cons"). The module PreludeList (see Section 8.1) defines many standard list functions. Arithmetic sequences and list comprehensions, two convenient syntaxes for special kinds of lists, are described in Sections 3.10 and 3.11, respectively. Lists are an instance of classes Read, Show, Eq, Ord, Monad, Functor, and MonadPlus.

6.1.4  Tuples

Tuples are algebraic datatypes with special syntax, as defined in Section 3.8. Each tuple type has a single constructor. All tuples are instances of Eq, Ord, Bounded, Read, and Show (provided, of course, that all their component types are).

There is no upper bound on the size of a tuple, but some Haskell implementations may restrict the size of tuples, and limit the instances associated with larger tuples. However, every Haskell implementation must support tuples up to size 15, together with the instances for Eq, Ord, Bounded, Read, and Show. The Prelude and libraries define tuple functions such as zip for tuples up to a size of 7.

The constructor for a tuple is written by omitting the expressions surrounding the commas; thus (x,y) and (,) x y produce the same value. The same holds for tuple type constructors; thus, (Int,Bool,Int) and (,,) Int Bool Int denote the same type.

The following functions are defined for pairs (2-tuples): fst, snd, curry, and uncurry. Similar functions are not predefined for larger tuples.

6.1.5  The Unit Datatype

data  () = () deriving (Eq, Ord, Bounded, Enum, Read, Show)

The unit datatype () has one non-_|_ member, the nullary constructor (). See also Section 3.9.

6.1.6  Function Types

Functions are an abstract type: no constructors directly create functional values. The following simple functions are found in the Prelude: id, const, (.), flip, ($), and until.

6.1.7  The IO and IOError Types

The IO type serves as a tag for operations (actions) that interact with the outside world. The IO type is abstract: no constructors are visible to the user. IO is an instance of the Monad and Functor classes. Chapter 7 describes I/O operations.

IOError is an abstract type representing errors raised by I/O operations. It is an instance of Show and Eq. Values of this type are constructed by the various I/O functions and are not presented in any further detail in this report. The Prelude contains a few I/O functions (defined in Section 8.3), and Part II contains many more.

6.1.8  Other Types

data  Maybe a     =  Nothing | Just a  deriving (Eq, Ord, Read, Show)
data  Either a b  =  Left a | Right b  deriving (Eq, Ord, Read, Show)
data  Ordering    =  LT | EQ | GT deriving
                                  (Eq, Ord, Bounded, Enum, Read, Show)

The Maybe type is an instance of classes Functor, Monad, and MonadPlus. The Ordering type is used by compare in the class Ord. The functions maybe and either are found in the Prelude.

6.2  Strict Evaluation

Function application in Haskell is non-strict; that is, a function argument is evaluated only when required. Sometimes it is desirable to force the evaluation of a value, using the seq function:

  seq :: a -> b -> b

The function seq is defined by the equations:

seq _|_b = _|_
seq a b = b, if a /=_|_

seq is usually introduced to improve performance by avoiding unneeded laziness. Strict datatypes (see Section 4.2.1) are defined in terms of the $! operator. However, the provision of seq has important semantic consequences, because it is available at every type. As a consequence, _|_ is not the same as \x ->  _|_, since seq can be used to distinguish them. For the same reason, the existence of seq weakens Haskell's parametricity properties.

The operator $! is strict (call-by-value) application, and is defined in terms of seq. The Prelude also defines the $ operator to perform non-strict application.

  infixr 0 $, $!
  ($), ($!) :: (a -> b) -> a -> b
  f $  x   =          f x
  f $! x   =  x `seq` f x

The non-strict application operator $ may appear 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 zipWith ($) fs xs.

6.3  Standard Haskell Classes

Figure 6.1 shows the hierarchy of Haskell classes defined in the Prelude and the Prelude types that are instances of these classes.
Diagram of standard Haskell classes

Figure 5

Standard Haskell Classes

Default class method declarations (Section 4.3) are provided for many of the methods in standard classes. A comment with each class declaration in Chapter 8 specifies the smallest collection of method definitions that, together with the default declarations, provide a reasonable definition for all the class methods. If there is no such comment, then all class methods must be given to fully specify an instance.

6.3.1  The Eq Class

  class  Eq a  where
        (==), (/=)  ::  a -> a -> Bool

        x /= y  = not (x == y)
        x == y  = not (x /= y)

The Eq class provides equality (==) and inequality (/=) methods. All basic datatypes except for functions and IO are instances of this class. Instances of Eq can be derived for any user-defined datatype whose constituents are also instances of Eq.

This declaration gives default method declarations for both /= and ==, each being defined in terms of the other. If an instance declaration for Eq defines neither == nor /=, then both will loop. If one is defined, the default method for the other will make use of the one that is defined. If both are defined, neither default method is used.

6.3.2  The Ord Class

  class  (Eq a) => Ord a  where
    compare              :: a -> a -> Ordering
    (<), (<=), (>=), (>) :: a -> a -> Bool
    max, min             :: a -> a -> a

    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

The Ord class is used for totally ordered datatypes. All basic datatypes except for functions, IO, and IOError, are instances of this class. 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.

The default declarations allow a user to create an Ord instance either with a type-specific compare function or with type-specific == and <= functions.

6.3.3  The Read and Show Classes

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

class  Read a  where
    readsPrec :: Int -> ReadS a
    readList  :: ReadS [a]
    -- ... default decl for readList given in Prelude

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

    showsPrec _ x s   = show x ++ s
    show x            = showsPrec 0 x ""
    -- ... default decl for showList given in Prelude

The Read and Show classes are used to convert values to or from strings. The Int argument to showsPrec and readsPrec gives the operator precedence of the enclosing context (see Section 10.4).

showsPrec and showList return a String-to-String function, to allow constant-time concatenation of its results using function composition. A specialised variant, show, is also provided, which uses precedence context zero, and returns an ordinary String. The method showList is provided to allow the programmer to give a specialised way of showing lists of values. This is particularly useful for the Char type, where values of type String should be shown in double quotes, rather than between square brackets.

Derived instances of Read and Show replicate the style in which a constructor is declared: infix constructors and field names are used on input and output. Strings produced by showsPrec are usually readable by readsPrec.

All Prelude types, except function types and IO types, are instances of Show and Read. (If desired, a programmer can easily make functions and IO types into (vacuous) instances of Show, by providing an instance declaration.)

For convenience, the Prelude provides the following auxiliary functions:

reads   :: (Read a) => ReadS a
reads   =  readsPrec 0

shows   :: (Show a) => a -> ShowS
shows   =  showsPrec 0

read    :: (Read a) => String -> a
read s  =  case [x | (x,t) <- reads s, ("","") <- lex t] of
              [x] -> x
              []  -> error " no parse"
              _   -> error " ambiguous parse"

and reads use a default precedence of 0. The read function reads input from a string, which must be completely consumed by the input process.

The function lex :: ReadS String, used by read, is also part of the Prelude. It 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 []).

6.3.4  The Enum Class

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]

    -- Default declarations given in Prelude

Class Enum defines operations on sequentially ordered types. The functions succ and pred return the successor and predecessor, respectively, of a value. The functions fromEnum and toEnum map values from a type in Enum to and from Int. The enumFrom... methods are used when translating arithmetic sequences (Section 3.10).

Instances of Enum may be derived for any enumeration type (types whose constructors have no fields); see Chapter 10.

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

The following Prelude types are instances of Enum:

For all four numeric types, succ adds 1, and pred subtracts 1. The conversions fromEnum and toEnum convert between the type and Int. In the case of Float and Double, the digits after the decimal point may be lost. It is implementation-dependent what fromEnum returns when applied to a value that is too large to fit in an Int.

For the types Int and Integer, the enumeration functions have the following meaning:

For Float and Double, the semantics of the enumFrom family is given by the rules for Int above, except that the list terminates when the elements become greater than e3+i/2 for positive increment i, or when they become less than e3+i/2 for negative i.

For all four of these Prelude numeric types, all of the enumFrom family of functions are strict in all their arguments.

6.3.5  The Functor Class

class  Functor f  where
    fmap    :: (a -> b) -> f a -> f b

The Functor class is used for types that can be mapped over. Lists, IO, and Maybe are in this class.

Instances of Functor should satisfy the following laws:

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

All instances of Functor defined in the Prelude satisfy these laws.

6.3.6  The Monad Class

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

    m >> k  =  m >>= \_ -> k
    fail s  = error s

The Monad class defines the basic operations over a monad. See Chapter 7 for more information about monads.

"do" expressions provide a convenient syntax for writing monadic expressions (see Section 3.14). The fail method is invoked on pattern-match failure in a do expression.

In the Prelude, lists, Maybe, and IO are all instances of Monad. The fail method for lists returns the empty list [], for Maybe returns Nothing, and for IO raises a user exception in the IO monad (see Section 7.3).

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

All instances of Monad defined in the Prelude satisfy these laws.

The Prelude provides the following auxiliary functions:

sequence  :: Monad m => [m a] -> m [a] 
sequence_ :: Monad m => [m a] -> m () 
mapM      :: Monad m => (a -> m b) -> [a] -> m [b]
mapM_     :: Monad m => (a -> m b) -> [a] -> m ()
(=<<)     :: Monad m => (a -> m b) -> m a -> m b

6.3.7  The Bounded Class

class  Bounded a  where
    minBound, maxBound :: a

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 types Int, Char, Bool, (), Ordering, and all tuples are instances of Bounded. 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.

6.4  Numbers

Haskell provides several kinds of numbers; the numeric types and the operations upon them have been heavily influenced by Common Lisp and Scheme. Numeric function names and operators are usually overloaded, using several type classes with an inclusion relation shown in Figure 6.1. The class Num of numeric types is a subclass of Eq, since all numbers may be compared for equality; its subclass Real is also a subclass of Ord, since the other comparison operations apply to all but complex numbers (defined in the Complex library). The class Integral contains integers of both limited and unlimited range; the class Fractional contains all non-integral types; and the class Floating contains all floating-point types, both real and complex.

The Prelude defines only the most basic numeric types: fixed sized integers (Int), arbitrary precision integers (Integer), single precision floating (Float), and double precision floating (Double). Other numeric types such as rationals and complex numbers are defined in libraries. In particular, the type Rational is a ratio of two Integer values, as defined in the Ratio library.

The default floating point operations defined by the Haskell Prelude do not conform to current language independent arithmetic (LIA) standards. These standards require considerably more complexity in the numeric structure and have thus been relegated to a library. Some, but not all, aspects of the IEEE floating point standard have been accounted for in Prelude class RealFloat.

The standard numeric types are listed in Table 6.1. The finite-precision integer type Int covers at least the range [ - 229, 229 - 1]. As Int is an instance of the Bounded class, maxBound and minBound can be used to determine the exact Int range defined by an implementation. Float is implementation-defined; it is desirable that this type be at least equal in range and precision to the IEEE single-precision type. Similarly, Double should cover IEEE double-precision. The results of exceptional conditions (such as overflow or underflow) on the fixed-precision numeric types are undefined; an implementation may choose error (_|_, semantically), a truncated value, or a special value such as infinity, indefinite, etc.

Type Class Description
Integer Integral Arbitrary-precision integers
Int Integral Fixed-precision integers
(Integral a) => Ratio a RealFrac Rational numbers
Float RealFloat Real floating-point, single precision
Double RealFloat Real floating-point, double precision
(RealFloat a) => Complex a Floating Complex floating-point

Table 2

Standard Numeric Types

The standard numeric classes and other numeric functions defined in the Prelude are shown in Figures 6.2--6.3. Figure 6.1 shows the class dependencies and built-in types that are instances of the numeric classes.

class  (Eq a, Show a) => Num a  where
    (+), (-), (*)  :: a -> a -> a
    negate         :: a -> a
    abs, 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, rem, div, mod :: a -> a -> a
    quotRem, 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, 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

Figure 6

Standard Numeric Classes and Related Operations, Part 1

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

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

gcd, 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

Figure 7

Standard Numeric Classes and Related Operations, Part 2

6.4.1  Numeric Literals

The syntax of numeric literals is given in Section 2.5. An integer literal represents the application of the function fromInteger to the appropriate value of type Integer. Similarly, a floating literal stands for an application of fromRational to a value of type Rational (that is, Ratio Integer). Given the typings:

fromInteger  :: (Num a) => Integer -> a
fromRational :: (Fractional a) => Rational -> a

integer and floating literals have the typings (Num a) => a and (Fractional a) => a, respectively. Numeric literals are defined in this indirect way so that they may be interpreted as values of any appropriate numeric type. See Section 4.3.4 for a discussion of overloading ambiguity.

6.4.2  Arithmetic and Number-Theoretic Operations

The infix class methods (+), (*), (-), and the unary function negate (which can also be written as a prefix minus sign; see section 3.4) apply to all numbers. The class methods quot, rem, div, and mod apply only to integral numbers, while the class method (/) applies only to fractional ones. The quot, rem, div, and mod class methods satisfy these laws if y is non-zero:

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

`quot` is integer division truncated toward zero, while the result of `div` is truncated toward negative infinity. The quotRem class method takes a dividend and a divisor as arguments and returns a (quotient, remainder) pair; divMod is defined similarly:

quotRem x y  =  (x 
`quot` y, x `rem` y)
divMod  x y  =  (x 
`div` y, x `mod` y)

Also available on integral numbers are the even and odd predicates:

even x =  x 
`rem` 2 == 0
odd    =  not . even

Finally, there are the greatest common divisor and least common multiple functions. 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 x y is the smallest positive integer that both x and y divide.

6.4.3  Exponentiation and Logarithms

The one-argument exponential function exp and the logarithm function log act on floating-point numbers and use base e. logBase a x returns the logarithm of x in base a. sqrt returns the principal square root of a floating-point number. There are three two-argument exponentiation operations: (^) raises any number to a nonnegative integer power, (^^) raises a fractional number to any integer power, and (**) takes two floating-point arguments. The value of x^0 or x^^0 is 1 for any x, including zero; 0**y is undefined.

6.4.4  Magnitude and Sign

A number has a magnitude and a sign. The functions abs and signum apply to any number and satisfy the law:

abs x * signum x == x

For real numbers, these functions are defined by:

abs x    | x >= 0  = x
         | x <  0  = -x

signum x | x >  0  = 1
         | x == 0  = 0
         | x <  0  = -1

6.4.5  Trigonometric Functions

Class Floating provides the circular and hyperbolic sine, cosine, and tangent functions and their inverses. Default implementations of tan, tanh, logBase, **, and sqrt are provided, but implementors are free to provide more accurate implementations.

Class RealFloat provides 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.

The precise definition of the above functions is as in Common Lisp, which in turn follows Penfield's proposal for APL [9]. See these references for discussions of branch cuts, discontinuities, and implementation.

6.4.6  Coercions and Component Extraction

The ceiling, floor, truncate, and round functions each take a real fractional argument and return an integral result. ceiling x returns the least integer not less than x, and floor x, the greatest integer not greater than x. truncate x yields the integer nearest x between 0 and x, inclusive. round x returns the nearest integer to x, the even integer if x is equidistant between two integers.

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 f with the same type and sign as x, and with absolute value less than 1. The ceiling, floor, truncate, and round functions can be defined in terms of properFraction.

Two functions convert numbers to type Rational: toRational returns the rational equivalent of its real argument with full precision; approxRational takes two real fractional arguments x and e and returns the simplest rational number within e of x, where a rational p/q in reduced form is simpler than another p' / q' if |p| <=|p'| and q <=q' . Every real interval contains a unique simplest rational; in particular, note that 0/1 is the simplest rational of all.

The class methods of class RealFloat allow efficient, machine-independent access to the components of a floating-point number. The functions floatRadix, floatDigits, and floatRange give the parameters of a floating-point type: the radix of the representation, the number of digits of this radix in the significand, and the lowest and highest values the exponent may assume, respectively. 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 mbn, where b is the floating-point radix, and furthermore, either m and n are both zero or else bd-1<=m<bd, where d is the value of floatDigits x. encodeFloat performs the inverse of this transformation. The functions significand and exponent together provide the same information as decodeFloat, but rather than an Integer, significand x yields a value of the same type as x, scaled to lie in the open interval (-1,1). exponent 0 is zero. scaleFloat multiplies a floating-point number by an integer power of the radix.

The functions isNaN, isInfinite, isDenormalized, isNegativeZero, and isIEEE all support numbers represented using the IEEE standard. For non-IEEE floating point numbers, these may all return false.

Also available are the following coercion functions:

fromIntegral :: (Integral a, Num b)    => a -> b
realToFrac   :: (Real a, Fractional b) => a -> b

The Haskell 98 Report
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December 2002