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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
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

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

:: 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
A specialised variant of `showsPrec`, using precedence context zero, and returning an ordinary `String`.
showList :: [a] -> ShowS
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 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 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 CULLong
Show CULong
Show CUShort
Show CUid
Show CWchar
Show CalendarTime
Show Char
Show ClockTime
Show Constr
Show ConstrRep
Show DataRep
Show DataType
Show Day
Show Doc
Show Double
Show Dynamic
Show Exception
Show ExitCode
Show Fd
Show Fixity
Show Float
Show GeneralCategory
Show Handle
Show HandlePosn
Show HandleType
Show HotLink
Show Html
Show HtmlAttr
Show HtmlTable
Show IOErrorType
Show IOException
Show IOMode
Show Int
Show Int16
Show Int32
Show Int64
Show Int8
Show IntSet
Show Integer
Show Lexeme
Show Month
Show Ordering
Show PackedString
Show Permissions
Show SeekMode
Show StdGen
Show ThreadId
Show TimeDiff
Show TimeLocale
Show TyCon
Show TypeRep
Show Version
Show Word
Show Word16
Show Word32
Show Word64
Show Word8
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 -> b)
Show a => Show (BlockTable a)
(RealFloat a, Show a) => Show (Complex a)
Show (ForeignPtr a)
Show (FunPtr a)
Show a => Show (IntMap a)
Show a => Show (Maybe a)
Show (Ptr a)
Integral a => Show (Ratio a)
Show a => Show (Set a)
Show a => Show (Tree a)
Show a => Show [a]
(Ix a, Show a, Show b) => Show (Array a b)
(Ix ix, Show ix, Show e) => Show (DiffArray ix e)
(Ix ix, Show ix) => Show (DiffUArray ix Char)
(Ix ix, Show ix) => Show (DiffUArray ix Double)
(Ix ix, Show ix) => Show (DiffUArray ix Float)
(Ix ix, Show ix) => Show (DiffUArray ix Int)
(Ix ix, Show ix) => Show (DiffUArray ix Int16)
(Ix ix, Show ix) => Show (DiffUArray ix Int32)
(Ix ix, Show ix) => Show (DiffUArray ix Int64)
(Ix ix, Show ix) => Show (DiffUArray ix Int8)
(Ix ix, Show ix) => Show (DiffUArray ix Word)
(Ix ix, Show ix) => Show (DiffUArray ix Word16)
(Ix ix, Show ix) => Show (DiffUArray ix Word32)
(Ix ix, Show ix) => Show (DiffUArray ix Word64)
(Ix ix, Show ix) => Show (DiffUArray ix Word8)
(Show a, Show b) => Show (Either a b)
(Show k, Show e) => Show (FiniteMap k e)
(Show k, Show a) => Show (Map k a)
Show (ST s a)
(Ix ix, Show ix) => Show (UArray ix Bool)
(Ix ix, Show ix) => Show (UArray ix Char)
(Ix ix, Show ix) => Show (UArray ix Double)
(Ix ix, Show ix) => Show (UArray ix Float)
(Ix ix, Show ix) => Show (UArray ix Int)
(Ix ix, Show ix) => Show (UArray ix Int16)
(Ix ix, Show ix) => Show (UArray ix Int32)
(Ix ix, Show ix) => Show (UArray ix Int64)
(Ix ix, Show ix) => Show (UArray ix Int8)
(Ix ix, Show ix) => Show (UArray ix Word)
(Ix ix, Show ix) => Show (UArray ix Word16)
(Ix ix, Show ix) => Show (UArray ix Word32)
(Ix ix, Show ix) => Show (UArray ix Word64)
(Ix ix, Show ix) => Show (UArray ix Word8)

shows :: Show a => a -> ShowS
equivalent to `showsPrec` with a precedence of 0.
showChar :: Char -> ShowS
utility function converting a `Char` to a show function that simply prepends the character unchanged.
showString :: String -> ShowS
utility function converting a `String` to a show function that simply prepends the string unchanged.
showParen :: Bool -> ShowS -> ShowS
utility function that surrounds the inner show function with parentheses when the `Bool` parameter is `True`.
showListWith :: (a -> ShowS) -> [a] -> ShowS
Show a list (using square brackets and commas), given a function for showing elements.
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