Show -package:rebase -package:rio -is:exact package:hedgehog
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)".
A specialised variant of
showsPrec, using precedence context
zero, and returning an ordinary
String.
Lifting of the
Show class to unary type constructors.
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.
utility function converting a
Char to a show function that
simply prepends the character unchanged.
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.
utility function that surrounds the inner show function with
parentheses when the
Bool parameter is
True.
utility function converting a
String to a show function that
simply prepends the string unchanged.
equivalent to
showsPrec with a precedence of 0.
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.
Annotates the source code with a value that might be useful for
debugging a test failure.
Logs a value to be displayed as additional information in the footer
of the failure report.