Haskell 98/2010 keeps a strict distinction between term-level syntax and tokens vs type-level. With ExplicitTypeApplications that's being eased: there's a lightweight way to sneak a type expression into a term. And ghc's internal term language 'core' has had explicit type applications for many years. Then consider a lightweight syntax using type applications in patterns to define typeclass instances by (pattern) 'matching' on types:
(==) @Int x y = primEqInt x y -- or eta-reduced (==) @Int = primEqInt -- shorthand for
instance Eq Int where (==) = primEqInt
Given that many typeclasses have a single method (or one base method with others defined in terms of it) [Note **], having to invent a typeclass name is a bit of a pain. Here's a lighweight method decl:
meth :: @a @b. => a -> b -> Bool -- typevar binding to left of => (there might also be superclass constraints) -- shorthand for
class Classfor_meth a b where -- generated class name meth :: a -> b -> Bool
Do we need class names at all? We could sneak term-level names into types. Just use the method name in constraints (with some handy syntactic marking -- what could be better than prefix @):
meth2 :: (@meth a b, @show b) => a -> b -> String meth2 x y = if (meth x y) then show y else "blah"
Of course we can infer the constraints on `meth2`, given its equation. Note **: for some of the Prelude's classes with many methods, like Num, there's long-standing complaints that the methods are too much tangled up and should be refactored into a hierarchy with a single method in each class: Additive, Multiplicative, Divisive, etc. With type families we can tackle the classic collections class (with methods empty, insert) as a superclass constraint , er, supermethod method:
type family Elem c :: * insert @c. => c -> Elem c -> c
empty :: @c. (@insert c) => c -- entangle empty with insert
Likewise for Monad:
(>>=) :: @m. => (m a) -> (a -> m b) -> (m b)
return :: @m. (@>>= m) => a -> m a
The soon-to-arrive Quantified Constraints extension will provide further ways to entangle methods/constraints. That might be an alternative way to express collections:
insert @c @e. (e ~ Elem c) => c -> e -> c
empty @c. (forall e. @insert c e => @empty c) => c
Of course all the above syntax is speculative/bikesheddable. The syntax needs to differentiate tyvars that are parameters/overloadable to the class/method vs parametric free tyvars. With explicit forall:
(>>=) :: forall m a b. @m. => (m a) -> (a -> m b) -> (m b)
return :: forall m a. @m. (@>>= m) => a -> m a
Or perhaps method signatures should look more like methods-as-constraints -- at cost of a bit of repetition
(>>=) :: forall m a b. (@>>= m) => (m a) -> (a -> m b) -> (m b)
return :: forall m a. (@return m, @>>= m) => a -> m a
AntC