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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 | 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`.

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

Show a list (using square brackets and commas), given a function for showing elements.
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