Impl Irrefutable Patterns in Match Exprs

[eignnx]

Currently Ast.Pat (irrefutable patterns) and Ast.RefutPat are totally separate. It would be nice to relate them so that an Ast.Pat can be used wherever an Ast.RefutPat is expected.

Could this be done using recursion schemes?

Ex:

data Pat r
  = VarPat String
  | TuplePat [r]

data RefutPat r
  = VarRefutPat String
  | VrntRefutPat String [r]
  | IrrefutPat  (Pat r)

Then let and match expressions would become:

data Expr
  ...
  | LetF (Pat (Fix Pat)) r
  ...
  | MatchF r [(RefutPat (Fix RefutPat), Seq r)]
  ...

Hopefully this would simplify pattern matching/destructuring compilation in TastToHir.hs.