Copyright | (c) The University of Glasgow 2001 |
---|---|

License | BSD-style (see the file libraries/base/LICENSE) |

Maintainer | libraries@haskell.org |

Stability | provisional |

Portability | portable |

Safe Haskell | Safe |

Language | Haskell2010 |

Converting values to readable strings:
the `Show`

class and associated functions.

# Documentation

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,

produces the string`show`

(Leaf 1 :^: Leaf 2 :^: Leaf 3)`"Leaf 1 :^: (Leaf 2 :^: Leaf 3)"`

.

:: Int | the operator precedence of the enclosing
context (a number from |

-> a | the value to be converted to a |

-> 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.

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.

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

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