| base-3.0.2.0: Basic libraries | Source code | Contents | Index

Text.Show

Portability | portable
Stability | provisional
Maintainer | libraries@haskell.org

Description
Converting values to readable strings: the `Show` class and associated functions.
Synopsis

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
showListWith :: (a -> ShowS) -> [a] -> ShowS

Documentation

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 `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 `Read` and `Show` satisfy the following:

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

That is, `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 All
Show Any
Show ArithException
Show ArrayException
Show AsyncException
Show Bool
Show BufferMode
Show CCc
Show CChar
Show CClock
Show CDev
Show CDouble
Show CFloat
Show CGid
Show CIno
Show CInt
Show CIntMax
Show CIntPtr
Show CLDouble
Show CLLong
Show CLong
Show CMode
Show CNlink
Show COff
Show CPid
Show CPtrdiff
Show CRLim
Show CSChar
Show CShort
Show CSigAtomic
Show CSize
Show CSpeed
Show CSsize
Show CTcflag
Show CTime
Show CUChar
Show CUInt
Show CUIntMax
Show CUIntPtr
Show CULLong
Show CULong
Show CUShort
Show CUid
Show CWchar
Show Char
Show Constr
Show ConstrRep
Show DataRep
Show DataType
Show Double
Show Dynamic
Show Exception
Show ExitCode
Show Fd
Show Fixity
Show Float
Show GeneralCategory
Show Handle
Show HandlePosn
Show HandleType
Show HashData
Show IOErrorType
Show IOException
Show IOMode
Show Int
Show Int16
Show Int32
Show Int64
Show Int8
Show IntPtr
Show Integer
Show Lexeme
Show Ordering
Show SeekMode
Show ThreadId
Show TyCon
Show TypeRep
Show Version
Show Word
Show Word16
Show Word32
Show Word64
Show Word8
Show WordPtr
Show ()
(Show a, Show b) => Show (a, b)
(Show a, Show b, Show c) => Show (a, b, c)
(Show a, Show b, Show c, Show d) => Show (a, b, c, d)
(Show a, Show b, Show c, Show d, Show e) => Show (a, b, c, d, e)
(Show 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)
Show (a -> b)
(RealFloat a, Show a) => Show (Complex a)
Show a => Show (Dual a)
Show a => Show (First a)
HasResolution a => Show (Fixed a)
Show (ForeignPtr a)
Show (FunPtr a)
Show a => Show (Last a)
Show a => Show (Maybe a)
Show a => Show (Product a)
Show (Ptr a)
Integral a => Show (Ratio a)
Show a => Show (Sum a)
Show a => Show [a]
(Ix a, Show a, Show b) => Show (Array a b)
(Show a, Show b) => Show (Either a b)
Show (ST s a)

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`.

showListWith :: (a -> ShowS) -> [a] -> ShowS | Source

Show a list (using square brackets and commas), given a function for showing elements.
Produced by Haddock version 0.9