Syntactical plugin to improve ergonomics

[L-as]

\x -> y -> maybePLam \x -> y
A x y z -> maybePCon $ A x y z
case x of { ... } -> maybePMatch x \case { ... }
Similarly for pattern guards, and other built-in syntax that can't be overloaded.

# Class-based solution
class MaybePLam x y out | out -> x y where
  maybePLam :: (x -> y) -> out

instance PLC e => MaybePLam (Term e a) (Term e b) (Term e (a #-> b)) where
  maybePLam = plam
  
instance {-# OVERLAPPABLE #-} PLC e => MaybePLam a b (a -> b) where
  maybePLam = id
  
# Type-family-based solution
type family F1 (x :: Type) :: Type where
  F1 (Term e (a #-> b)) = Term e a
  F1 (a -> b) = a

type family F2 (x :: Type) :: Type where
  F2 (Term e (a #-> b)) = Term e b
  F2 (a -> b) = b

type family IsTermFunc (x :: Type) :: Bool where
  F2 (Term e (_ #-> _)) = 'True
  F2 (a -> b) = 'False

maybePLam :: KnownBool (IsTermFunc out) => (F1 out -> F2 out) -> out
maybePLam = ...

[L-as]

phadej/overloaded#44