#lang rhombus/static

// Exercises (start the first and last by writing an initially-failing test case):
//  * Add a subtraction operation
//  * Change `let` to rewrite to an immediately called `fun`
//  * Add an `if` (conditional) form

fun interp(e :: Syntax, env :: Map):
  match e
  | '($inside ...)':
      interp('$inside ...', env)
  | '$(name :: Identifier)':
      if name.unwrap() in env
      | env[name.unwrap()]
      | error("free variable " +& name)
  | '$left ... + $right ...':
      interp('$left ...', env) + interp('$right ...', env)
  | '$left ... == $right ...':
      interp('$left ...', env) == interp('$right ...', env)
  | 'let $(name :: Identifier) = $rhs ...: $body ...':
      interp('$body ...',
             env ++ { name.unwrap(): interp('($rhs ...)', env) })
  | 'fun ($(arg :: Identifier), ...): $body ...':
      fun (arg_val, ...): interp('$body ...',
                                 env ++ { arg.unwrap(): arg_val, ... })
  | '$fun ... ($arg ..., ...)':
      interp('$fun ...', env)(interp('$arg ...', env), ...)
  | '$(e :: Term)':
      e.unwrap()

check: interp('let f = (fun (x): x + x):
                 f(7)', {})
       ~is 14

check: interp('(fun (x, y): x == y)(7, 8)', {})
       ~is #false

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