Extending a datatype in Haskell

Question

Haskell newbie here.

I wrote an evaluator for a minimal assembly-like language.

Now, I want to extend that language to support some syntactic sugar which,

Answer 1

A (simpler) option is to add a type to your AST, to distinguish Core from Extended:

data Core = Core
data Extended = Extended

data Expr t 
  = Add (Expr t) (Expr t)
  | Mult (Expr t) (Expr t)
  | Const Int 
  | Sugar t (Expr t) (Expr t)

An expression is either Core or Extended: the compiler will ensure that it contains only sub-expressions of the same type.

The function signatures in your original module would need to use Expr Core (instead of just Expr)

A Desugar function would have the following type signature:

Desugar :: Expr Extended -> Expr Core

You may also be interested in the more sophisticated approach described in the paper 'Trees that grow'.