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 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 toplevel 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 recordsyntax 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 rightassociativity of :^: is ignored. For example,
 show (Leaf 1 :^: Leaf 2 :^: Leaf 3) produces the string
"Leaf 1 :^: (Leaf 2 :^: Leaf 3)".
  Methods  showsPrec  :: 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  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.

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