Staged Interpreter

gistfile1.dart

class None {
  None();
}

class Some {
  final value;
  Some(this.value);
}

/* environments */

class ConsEnv {
  final x;
  final value;
  final tail;
  ConsEnv(this.x, this.value, this.tail);
  
  insert(x, v) => new ConsEnv(x, v, this);
  lookup(x) => (x == this.x) ? new Some(this.value) : this.tail.lookup(x);
}

class NilEnv {
  NilEnv();
  
  insert(x, v) => new ConsEnv(x, v, this);
  lookup(x) => new None();
}

/* AST + interpreter */

class Lit {
  final value;
  Lit(this.value);
  
  eval(env) => lift(this.value);
}

class Var {
  final x;
  Var(this.x);
  
  eval(env) => env.lookup(this.x).value;
}

class Lambda {
  final x;
  final t;
  Lambda(this.x, this.t);
  
  eval(env) => <(y) => ~this.t.eval(env.insert(this.x, <y>))>;
}

class App {
  final t1;
  final t2;
  App(this.t1, this.t2);
  
  eval(env) => <(~this.t1.eval(env))(~this.t2.eval(env))>;
}

main() {
  final f = new Lambda("x", new Lambda("y", new Var("x")));
  final e = new App(new App(f, new Lit(42)), new Lit(43));
  print(e.eval(new NilEnv()));
}