http://www.paulgraham.com/arclessons.html

Skip to "Implicit local variables conflict with macros."

As for implicit return statements, I don't know what you expect me to be, but I'm for them. ^_^ I think you can always say "return undefined;" in JavaScript to accomplish the same thing as leaving out the return statement, so in a way, leaving the return statement out is just syntactic sugar.

I'm for explicit return statements too, for the purposes of short-circuiting. However, they probably won't play well with macros, for the same reason as pg encountered with implicit local variables: Lexical scope boundaries might pop up in unexpected places and change the meaning of "return;".

> Or is it really naive to think it could work out this way? :P

Well, I do have lots of suggestions. They go pretty deep into the fundamental design too. That said, they sort of lead to not programming in Arc. You'll see what I mean in a second (unless I go scatterbrained on ya again).

I think it would be best to treat this as a combinator library. You can have utilities that build not only JavaScript expressions and statements,* but also other intermediate forms, like lists of statements. I wouldn't be surprised to see http://en.wikipedia.org/wiki/Control_flow_graph pop up at some point. As the combinator library goes on, it can introduce more convenient construction syntaxes, including full DSLs.

If this is beginning to sound similar to the topic I just posted, "A Language Should Target Moving Platforms"[1], that's because it is. We're talking about generating JavaScript code. It's very closely related to the topic of generating code in general. :-p

I was developing Penknife not too long ago. It's a language much like Arc, but what's important here is that its expressions are compiled and macro-expanded to an intermediate format first--i.e. parsed into an AST--and it's easy to make closures whose ASTs can be inspected. My plan is for the Penknife AST itself to be extensible using user-defined types, and my goal is for non-Penknife ASTs to be made using the same syntax technology used elsewhere in Penknife, right down to using the same compiler.

So yeah, this is my apprach to a JavaScript DSL. ^_^ I don't expect you to go adopt the whole approach, but if you do want to pursue it, or if there's some aspect of it that would fit into your design too, be my guest.

* Fine-grained control is fine by me. :-p So is automagic. I want it all!

I couldn't believe this, but I looked around and yes, it's true, "..the scope container in javascript is a function." (http://mark-story.com/posts/view/picking-up-javascript-closu...)

That seems bad. Don't create a new function in js everytime I do a let in arc. But I don't know if you can avoid doing so.

Yep! Function scope + no tail-call optimization = a formidable challenge. ^_^

> Perhaps one way to do it would be to play a little fast and loose with semantics. Assume let translates to the scope of the containing function.

Interesting approach. I'll play around with this. There might also be weird hacks that could simulate let without costing a whole function on the stack. For example, you might be able to get away with something like this:

  (let x 5         var _temp = true;
    ... )          while (_temp) {
                     _temp = false;
                     var x = 5;
                     ...
                   }

  (let x 5         var _temp = true;
    ... )          for (var x = 5; _temp = false; _temp)                           
                     ...
                   }

  (let x 5         (function() {
    ... )            var x = 5;
                     ...
                   }).call(this);

  var x = 1;
  {
      var x = 2;
      var y = "but why!?";
      alert( x );           // 2
  }
  alert( x );               // 2
  alert( y );               // but why!?
  
  var x = 1;
  do
  {
      var x = 2;
      var y = "but why!?";
      alert( x );           // 2
  }
  while( false );
  alert( x );               // 2
  alert( y );               // but why!?

For a high-level (and non-idiomatic) approach, it's possible to implement trampolining using exceptions. I've done it in about a page of JavaScript (right next to my Fermat's Last Theorem proof, of course of course :-p ), and I think the API can be as simple as this:

  - Use bounce( foo, foo.bar, 1, 2 ,3 ) instead of
      foo.bar( 1, 2, 3 ) in tail positions. This throws an
      exception containing information about the next call
      to make.
  - To begin a trampolining loop, call the function with
      trampoline( foo, foo.bar, 1, 2, 3 ). This should be
      done when calling functions at the top level and when
      calling functions from within functions that are to be
      exposed to other JavaScript utilities and DOM event
      handlers (which won't call trampoline themselves).

I think this could work pretty nicely, actually:

  (let x 5         var _x;
    ... )          _x = 5;
                   ...

  (= x 6)            var x, _x;
  (let x 5           x = 6;
    (alert x))       
  (alert x)          _x = 5;
                     alert(_x); // => 5

                     alert(x); // => 6

  (let x 5           var _x, __x;
    (alert x))         
  (let x 6           _x = 5;
    (alert x))       alert(_x); // => 5

                     __x = 6;
                     alert(__x); // => 6

The underscore prefix system would be nice because it's pretty readable, but it's not necessary. In the worst case, we can use gensyms. I thought using underscores for internal variables might work because CoffeeScript does it [1], but maybe it won't. So you raise a good point. :)

> Also, will the name "let!" conflict with ssyntax?

Yes. I'm starting from scratch with the ssyntax. `!` is a regular character now. It can be used in names like `let!`, and when by itself in functional position it compiles to the JavaScript negation operator:

  (! x)   =>   !x

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[1] http://jashkenas.github.com/coffee-script/#comprehensions