Text.Show

Copyright (c) The University of Glasgow 2001
License BSD-style (see the file libraries/base/LICENSE)
Maintainer [email protected]
Stability provisional
Portability portable
Safe Haskell Safe
Language Haskell2010

Description

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

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 Strings.

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)".

Minimal complete definition

showsPrec | show

Methods

showsPrec Source

Arguments

:: 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:

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 Bool
Show Char
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 CallStack
Show TypeRep
Show ()
Show SomeException
Show Number
Show Lexeme
Show GeneralCategory
Show Fingerprint
Show TyCon
Show Associativity
Show Fixity
Show Arity
Show Any
Show All
Show ArithException
Show ErrorCall
Show IOException
Show MaskingState
Show Dynamic
Show CUIntMax
Show CIntMax
Show CUIntPtr
Show CIntPtr
Show CSUSeconds
Show CUSeconds
Show CTime
Show CClock
Show CSigAtomic
Show CWchar
Show CSize
Show CPtrdiff
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 IntPtr
Show WordPtr
Show CodingProgress
Show TextEncoding
Show SeekMode
Show NewlineMode
Show Newline
Show BufferMode
Show Handle
Show IOErrorType
Show ExitCode
Show ArrayException
Show AsyncException
Show SomeAsyncException
Show AssertionFailed
Show AllocationLimitExceeded
Show Deadlock
Show BlockedIndefinitelyOnSTM
Show BlockedIndefinitelyOnMVar
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 CodingFailureMode
Show ThreadStatus
Show BlockReason
Show ThreadId
Show NestedAtomically
Show NonTermination
Show NoMethodError
Show RecUpdError
Show RecConError
Show RecSelError
Show PatternMatchFail
Show IOMode
Show Lifetime
Show Event
Show FdKey
Show HandlePosn
Show GCStats
Show Version
Show Fixity
Show ConstrRep
Show DataRep
Show Constr
Show DataType
Show Natural
Show RTSFlags
Show TickyFlags
Show TraceFlags
Show DoTrace
Show ProfFlags
Show DoHeapProfile
Show CCFlags
Show DoCostCentres
Show DebugFlags
Show MiscFlags
Show ConcFlags
Show GCFlags
Show GiveGCStats
Show SomeSymbol
Show SomeNat
Show SrcLoc
Show StaticPtrInfo
Show Void
Show a => Show [a]
(Integral a, Show a) => Show (Ratio a)
Show (Ptr a)
Show (FunPtr a)
Show (U1 p)
Show p => Show (Par1 p)
Show a => Show (Maybe a)
Show a => Show (Down 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 (ForeignPtr a)
Show a => Show (ZipList a)
Show a => Show (Complex a)
HasResolution a => Show (Fixed a)
Show a => Show (Identity a)

This instance would be equivalent to the derived instances of the Identity newtype if the runIdentity field were removed

(Show a, Show b) => Show (Either a b)
Show (f p) => Show (Rec1 f p)
(Show a, Show b) => Show (a, b)
Show (ST s a)
Show (Proxy k s)
Show a => Show (Const a b)
Show c => Show (K1 i c p)
(Show (f p), Show (g p)) => Show ((:+:) f g p)
(Show (f p), Show (g p)) => Show ((:*:) f g p)
Show (f (g p)) => Show ((:.:) f g p)
(Show a, Show b, Show c) => Show (a, b, c)
Show ((:~:) k a b)
Show (Coercion k a b)
Show (f a) => Show (Alt k f a)
Show (f p) => Show (M1 i c f p)
(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.

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/7.10.3/docs/html/libraries/base-4.8.2.0/Text-Show.html