base-4.2.0.2: Basic librariesContentsIndex
Text.Show
Portabilityportable
Stabilityprovisional
Maintainerlibraries@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 Strings.

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
:: Intthe operator precedence of the enclosing context (a number from 0 to 11). Function application has precedence 10.
-> athe 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
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.
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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