expr-eval.ns

external context : top := "Record<string, number>"
external context-empty : context := "{}"
external context-get(ctx : context, key : string) : number := "ctx[key]"
external context-set(ctx : context, key : string, val : number) : context :=
  "(Omit<ctx, key> & Record<key, val>)"

enum expr :=
| lit(value : number)
| ref(variable : string)
| add(a b : expr)
| local(variable : string, value next : expr)

def eval(ctx : context, e : expr) : number :=
  match e with
  | lit(x) := x
  | ref(x) := context-get(ctx, x)
  | add(a, b) := plus(eval(ctx, a), eval(ctx, b))
  | local(name, value, next) :=
      let (evaled-value : number) := eval(ctx, value)
          new-context := context-set(ctx, name, evaled-value)
       in eval(new-context, next)
  end

def pure(e : expr) : number :=
  eval(context-empty, e)

def a : number :=
  pure(local("a", add(lit(1), lit(20)), add(ref("a"), ref("a"))))