Deriving via first-class functions

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deriving-via-fun is a library for deriving instances by treating classes like functors.

First, define some type, for example:

data T0 = T0 Int Bool

We can reuse existing instances for (Int, Bool) to derive instances for T0, by providing a function T0 -> (Int, Bool). A handy one you can find in the library is the “generic isomorphism” T0 ?-> (Int, Bool).

deriving (Eq, Ord) via Fun (T0 ?-> (Int, Bool))

For another example, let’s derive the All monoid (aka. (&&)) from Any (aka. (||)) by duality:

newtype All = All Bool
  deriving (Semigroup, Monoid)
    via Fun (Coerce All Bool >>> Not >>> Coerce Bool Any)

deriving-via-fun is available on Hackage. See the README and the haddocks for more details.

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This is a single-module package with no dependencies other than base. Yet things like GenericIso, or categories of isomorphisms in general, could be very useful by itself. Please consider splitting into several packages.