Data.Poly.Functor

Documentation

class PolyFunctor t where

A very generic class for a map function on heterogeneous data structures (i.e. those with differing types).

This allows you to do things like:

>>> hmap triple (3 :: Int, 4.5 :: Float)
(9 :: Int, 13.5 :: Float)

hmap takes as it's function a Poly, as of course you'd want a polymorphic function.

The return type defined in the class is very vague, indeed it's just t to be detailed in the instances, because unlike a normal map function, how hmap changes the type depends a lot on the type it's applied to, there's no simple f a -> f b.

Currently only instances are defined are for 2 and 3 tuples, nag me if you want larger ones.

It's worth noting how the instances are defined, for example, for the 2 tuples, there are 3 instances defined. This is primarily to help type inference. We don't know too much about the types hmap will produce, but we do know, if we feed hmap a pair, we should get a pair back. Likewise, if the result of hmap is a pair, then the input should be a pair.

So we provide both instances where the input is a pair, and when the output is a pair. In both of these instances, we then in the constraints section (which happens after instance selection) ensure the other argument is also a pair.

The "know both are pairs already" case just needs to be added as a specific overlapping instance so the compiler has a most specific match when it already knows both input and output are pairs.

Minimal complete definition

hmap

Methods

hmap :: forall c. PolyFunctorConstraint c t => Poly c -> t

Instances

PolyFunctor ((a0, a1) -> (b0, b1))

Methods

hmap :: PolyFunctorConstraint c ((a0, a1) -> (b0, b1)) => Poly c -> (a0, a1) -> (b0, b1)

IsTuple 2 b => PolyFunctor ((a0, a1) -> b)

Methods

hmap :: PolyFunctorConstraint c ((a0, a1) -> b) => Poly c -> (a0, a1) -> b

PolyFunctor ((a0, a1, a2) -> (b0, b1, b2))

Methods

hmap :: PolyFunctorConstraint c ((a0, a1, a2) -> (b0, b1, b2)) => Poly c -> (a0, a1, a2) -> (b0, b1, b2)

IsTuple 3 b => PolyFunctor ((a0, a1, a2) -> b)

Methods

hmap :: PolyFunctorConstraint c ((a0, a1, a2) -> b) => Poly c -> (a0, a1, a2) -> b

IsTuple 3 a => PolyFunctor (a -> (b0, b1, b2))

Methods

hmap :: PolyFunctorConstraint c (a -> (b0, b1, b2)) => Poly c -> a -> (b0, b1, b2)

IsTuple 2 a => PolyFunctor (a -> (b0, b1))

Methods

hmap :: PolyFunctorConstraint c (a -> (b0, b1)) => Poly c -> a -> (b0, b1)

hmap :: forall c. (PolyFunctor t, PolyFunctorConstraint c t) => Poly c -> t

type family PolyFunctorConstraint (c :: * -> Constraint) t :: Constraint

Instances

type PolyFunctorConstraint c ((a0, a1, a2) -> (b0, b1, b2))

type PolyFunctorConstraint c ((a0, a1, a2) -> (b0, b1, b2)) = (c (a0 -> b0), c (a1 -> b1), c (a2 -> b2))

type PolyFunctorConstraint c ((a0, a1) -> (b0, b1))

type PolyFunctorConstraint c ((a0, a1) -> (b0, b1)) = (c (a0 -> b0), c (a1 -> b1))