Development.Shake.Classes

Conversion of values to readable Strings.

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)".

The class Typeable allows a concrete representation of a type to be calculated.

The Eq class defines equality (==) and inequality (/=). All the basic datatypes exported by the Prelude are instances of Eq, and Eq may be derived for any datatype whose constituents are also instances of Eq.

The Haskell Report defines no laws for Eq. However, instances are encouraged to follow these properties:

Reflexivity

x == x

=

True

Symmetry

x == y

=

y == x

Transitivity

if

x == y && y == z

=

True

, then

x == z

=

True

Extensionality

if

x == y

=

True

and

f

is a function whose return type is an instance of

Eq

, then

f x == f y

=

True

Negation

x /= y

=

not (x == y)

The class of types that can be converted to a hash value.

Minimal implementation: hashWithSalt.

Hashable is intended exclusively for use in in-memory data structures. . Hashable does not have a fixed standard. This allows it to improve over time. . Because it does not have a fixed standard, different computers or computers on different versions of the code will observe different hash values. As such, Hashable is not recommended for use other than in-memory datastructures. Specifically, Hashable is not intended for network use or in applications which persist hashed values. For stable hashing use named hashes: sha256, crc32, xxhash etc.

If you are looking for Hashable instance in time package, check time-compat