a tiny forth interpreter

forth.dart

import 'package:flutter/material.dart';

void main() {
  Forth().run('V: count 5 ! : inc dup @ 1 + ! ; '
      '[ [ text ] count builder [ count inc ] " Increment text button ] '
      'list column app run');
}

typedef Impl = void Function(Forth f);

class Forth {
  final stack = <dynamic>[];
  void push(dynamic value) => stack.add(value);
  dynamic pop() => stack.removeLast();

  List<String> words = [];
  String next() => words.removeAt(0);

  List<Impl>? code;
  final Map<String, Impl> vocab = {
    '+': (f) => f.push((f.pop() as num) + (f.pop() as num)),
    '.': (f) => print(f.pop()),
    'dup': (f) => f.push(f.stack.last),
    ':': (f) {
      final name = f.next();
      final code = <Impl>[];
      f.vocab[name] = (f) => f.exec(code);
      f.code = code;
    },
    ';': (f) => f.code = null,
    '"': (f) => f.add(f.next()),
    'V:': (f) {
      final name = f.next();
      final value = ValueNotifier<dynamic>(null);
      f.vocab[name] = f.lit(value);
      f.push(value);
    },
    '@': (f) => f.push((f.pop() as ValueNotifier<dynamic>).value),
    '!': (f) => ((dynamic v) => (f.pop() as ValueNotifier<dynamic>).value = v)(f.pop()),
    '[': (f) {
      f.push(f.code);
      f.code = <Impl>[];
    },
    ']': (f) {
      final quot = f.code;
      if (quot == null) throw Exception('] without [');
      f.code = f.pop() as List<Impl>?;
      f.add(quot);
    },
    'list': (f) {
      final i = f.stack.length - 1;
      f.exec(f.pop() as List<Impl>);
      final l = f.stack.sublist(i);
      f.stack.length = i;
      f.push(l);
    },
    'builder': (f) {
      final vl = f.pop() as ValueNotifier<dynamic>;
      final quot = f.pop() as List<Impl>;
      f.push(ValueListenableBuilder(
        valueListenable: vl,
        builder: (context, value, _) {
          f.push(value);
          f.exec(quot);
          return f.pop() as Widget;
        },
      ));
    },
    'text': (f) => f.push(Text('${f.pop()}')),
    'button': (f) {
      final child = f.pop() as Widget;
      final quot = f.pop() as List<Impl>;
      f.push(ElevatedButton(onPressed: () => f.exec(quot), child: child));
    },
    'column': (f) => f.push(Column(children: (f.pop() as List).cast<Widget>())),
    'app': (f) => f.push(MaterialApp(home: Scaffold(body: f.pop() as Widget))),
    'run': (f) => runApp(f.pop() as Widget),
  };
  final immediate = {';', '"', '[', ']'};

  void run(String input) {
    words = input.split(' ');
    while (words.isNotEmpty) {
      final word = next();
      final impl = vocab[word] ?? lit(num.parse(word));
      if (immediate.contains(word) || code == null) {
        impl(this);
      } else {
        code!.add(impl);
      }
    }
  }

  Impl lit(dynamic value) => (f) => f.push(value);
  void add(dynamic value) => code != null ? code!.add(lit(value)) : push(value);
  void exec(List<Impl> quot) {
    for (final impl in quot) {
      impl(this);
    }
  }
}