Editing Generic programming (section)

=====Generic Haskell=====
Generic Haskell is another extension to [[Haskell]], developed at [[Utrecht University]] in the [[Netherlands]]. The extensions it provides are:
*''Type-indexed values'' are defined as a value indexed over the various Haskell type constructors (unit, primitive types, sums, products, and user-defined type constructors). In addition, we can also specify the behaviour of a type-indexed values for a specific constructor using ''constructor cases'', and reuse one generic definition in another using ''default cases''.
The resulting type-indexed value can be specialized to any type.
*''Kind-indexed types'' are types indexed over kinds, defined by giving a case for both ''*'' and ''k → k'''. Instances are obtained by applying the kind-indexed type to a kind.
*Generic definitions can be used by applying them to a type or kind. This is called ''generic application''. The result is a type or value, depending on which sort of generic definition is applied.
*''Generic abstraction'' enables generic definitions be defined by abstracting a type parameter (of a given kind).
*''Type-indexed types'' are types that are indexed over the type constructors. These can be used to give types to more involved generic values. The resulting type-indexed types can be specialized to any type.

As an example, the equality function in Generic Haskell:<ref>[https://www.cs.uu.nl/research/projects/generic-haskell/compiler/diamond/GHUsersGuide.pdf The Generic Haskell User's Guide]</ref>

<syntaxhighlight lang="haskell">
   type Eq {[ * ]} t1 t2 = t1 -> t2 -> Bool
   type Eq {[ k -> l ]} t1 t2 = forall u1 u2. Eq {[ k ]} u1 u2 -> Eq {[ l ]} (t1 u1) (t2 u2)

   eq {| t :: k |} :: Eq {[ k ]} t t
   eq {| Unit |} _ _ = True
   eq {| :+: |} eqA eqB (Inl a1) (Inl a2) = eqA a1 a2
   eq {| :+: |} eqA eqB (Inr b1) (Inr b2) = eqB b1 b2
   eq {| :+: |} eqA eqB _ _ = False
   eq {| :*: |} eqA eqB (a1 :*: b1) (a2 :*: b2) = eqA a1 a2 && eqB b1 b2
   eq {| Int |} = (==)
   eq {| Char |} = (==)
   eq {| Bool |} = (==)
</syntaxhighlight>