#lang rhombus/static

// Note: this kind of interpreter is not what we have in mind
// by "metaprogramming in Rhombus". It's just an example in
// a domain that we expect to be familiar to participants.

// Exercises:
//  * Add a `Minus` expression form; see the
//    commented-out test at the end
//  * Add an `If` expression form; see the
//    commented-out test at the end

class Expr():
  nonfinal

class Id(name :: Symbol):
  extends Expr

class Plus(left :: Expr,
           right :: Expr):
  extends Expr

class Equals(left :: Expr,
             right :: Expr):
  extends Expr

class Let(name :: Symbol,
          rhs :: Expr,
          body :: Expr):
  extends Expr

class Fun(arg :: Symbol,
          body :: Expr):
  extends Expr

class Call(fun :: Expr,
           arg :: Expr):
  extends Expr

class Literal(val :: Any):
  extends Expr

fun interp(e :: Expr, env :: Map):
  match e
  | Id(name):
      if name in env
      | env[name]
      | error("free variable " +& name)
  | Plus(left, right):      
      interp(left, env) + interp(right, env)
  | Equals(left, right):      
      interp(left, env) == interp(right, env)
  | Let(name, rhs, body):
      interp(body, env ++ { name: interp(rhs, env) })
  | Fun(arg, body):
      fun (arg_val): interp(body, env ++ { arg: arg_val })
  | Call(fun, arg):
      interp(fun, env)(interp(arg, env))
  | Literal(val):
      val

check: interp(Id(#'x), { #'x: 5})
       ~is 5

check: interp(Let(#'f,
                  Fun(#'x, Plus(Id(#'x), Id(#'x))),
                  Call(Id(#'f), Literal(7))),
              {})
       ~is 14

check: interp(Call(Call(Fun(#'x,
                            Fun(#'y,
                                Equals(Id(#'x), Id(#'y)))),
                        Literal(7)),
                   Literal(8)),
              {})
       ~is #false

check: interp(Id(#'x), {})
       ~throws "free variable x"

/*
check: interp(Minus(Literal(10), Literal(1)), {})
       ~is 9
*/

/*
check: interp(If(Equals(Literal(0), Id(#'x)),
                 Literal(2),
                 Literal(3)),
              { #'x: 5 })
       ~is 3
*/