Data.Poly.Function

Documentation

The easiest way to use these mkPoly* functions is by adding the extension:

\{\-# LANGUAGE TypeApplications #\-\}

at the top of your source file (GHC Documentation on the Type Application extension).

Examples of this will be included in the documentation for each convience function.

Note that all of these convience functions are just type restricted versions of Poly, that's all, and are all defined in this form:

f = Poly

Also not that this is far from an exhaustive list of what can be done, there's a more general approach described in the documentation for Poly

mkPolyHomoFunc1 :: forall c. (forall t. c t => t -> t) -> Poly (IsHomoFunc 1 &&& (Arg 0 `IxConstrainBy` c))

'mkPolyHomoFunc1 simply represents a function from t -> t, possibly constrained.

For example, this is how to write a polymorphic version of "triple":

mkPolyHomoFunc1 @Num (*3)

mkPolyFunc1 :: forall c1 c2. (forall t1 t2. (c1 t1, c2 t2) => t1 -> t2) -> Poly ((IsFunc 1 &&& (Arg 0 `IxConstrainBy` c1)) &&& (Result 1 `IxConstrainBy` c2))

'mkPolyFunc1 is for one argument functions with differing arguments.

For example, this is how to write a polymorphic version of toInteger:

mkPolyFunc1 @Integral @(Equal Integer) toInteger

Note that something like Just :: t -> Maybe t this convience function is not helpful for, because the two constraints you pass here are separate.

mkPolyHomoFunc2 :: forall c. (forall t. c t => t -> t -> t) -> Poly (IsHomoFunc 2 &&& (Arg 0 `IxConstrainBy` c))

'mkPolyHomoFunc2 simply represents a function from t -> t -> t, possibly constrained.

For example, this is how to write a polymorphic version of "add":

mkPolyHomoFunc2 @Num (+)

mkPolyHomoArgFunc2 :: forall c1 c2. (forall t1 t2. (c1 t1, c2 t2) => t1 -> t1 -> t2) -> Poly ((IsHomoArgFunc 2 &&& (Arg 0 `IxConstrainBy` c1)) &&& (Result 2 `IxConstrainBy` c2))

'mkPolyArgFunc2 represents a function from t -> t -> r, with two constraints, one for the arguments, one for the result.

For example, this is how to write a polymorphic version of "eq":

mkPolyHomoArgFunc2 @Eq @(Equal Bool) (==)

Below are some convience constraints that make it easier to write polymorphic functions.

class a ~ b => Equal a b

Handy type class for expressing an "is equal to" constraint, because as a class it can be partially applied.

For example, whilst Num is a constraint function from (* -> Constraint) such that (Num t) succeeds only if t is a Num, Equal Int is a constraint function such that (Equal Int) t succeeds only if t is an Int.

For example:

mkPolyFunc1 @Integral @(Equal Integer) toInteger

Is a polymorphic toInteger function.

Instances

(~) * a b => Equal a b

class Empty a

The empty constraint:

Empty a

always succeeds.

Instances

Empty a