Optional tokens

[eknkc]

Hi, Ive been trying to implement a "def" macro that supports default parameters. What I could come up with is this:

macro def {
    case ($($p:ident = $v:expr) (,) ...) { $body:SourceElements } => {
        def($p (,) ...) {
            $($p = typeof $p === 'undefined' ? $v : $p) (;) ...;
            $body
        };
    }

    case ($params:ident (,) ...) { $body:SourceElements } => {
        $(function($params (,) ...) {
            $body
        });
    }
}

So, it works for def(a, b) { } and def(a = 4, b = a) { } but not def(a, b = 4) { }. What would be a good way to do this? I thought if I could denote the = $v:expr part as optional, it would be ok but have no idea about the syntax.

Is there a better way to do this? Maybe a recursive solution?

[disnet]

I think what you might want is some helper macros:

macro helper_params_names {
  // extract just the param names
}

macro helper_assign_default {
  // do default assignment
}

macro def {
    case ($params ...) { $body } => {
      (function(helper_param_names($params ...)) {
        helper_assign_default($params ...);
        $body
      });
    }
}

[eknkc]

I receive this error when I try to place a macro in function parameter definitions:

15    case ($params:ident (,) ...) { $body:SourceElements } => {
16        (function(helper_expand_params($params (,) ...)) {
17            $body
18        });
19    }

Error: Line 16: Unexpected token (
    at throwError (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1172:21)
    at throwUnexpected (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1233:9)
    at parseVariableIdentifier (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:2172:13)
    at parseFunctionExpression (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:3062:25)
    at parsePrimaryExpression (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1540:24)
    at parseLeftHandSideExpressionAllowCall (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1734:48)
    at parsePostfixExpression (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1781:20)
    at parseUnaryExpression (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1851:16)
    at parseMultiplicativeExpression (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1857:20)
    at parseAdditiveExpression (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1874:20)

[disnet]

Can you post your entire code?

[eknkc]

I did not went on to recursive param helpers but this much should work as far as I can tell;

macro expand_params {
    case ($param ...) => {
        $param ...
    }
}

macro def {
    case ($params ...) { $body:SourceElements } => {
        (function(expand_params($params ...)) {
            $body
        });
    }
}

def(a, b) {
    return a + b;
}

/usr/local/lib/node_modules/sweet.js/lib/sweet.js:5038
            throw e;
                  ^
Error: Line 9: Unexpected token (
    at throwError (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1172:21)
    at throwUnexpected (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1233:9)
    at parseVariableIdentifier (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:2172:13)
    at parseFunctionExpression (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:3062:25)
    at parsePrimaryExpression (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1540:24)
    at parseLeftHandSideExpressionAllowCall (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1734:48)
    at parsePostfixExpression (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1781:20)
    at parseUnaryExpression (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1851:16)
    at parseMultiplicativeExpression (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1857:20)
    at parseAdditiveExpression (/usr/local/lib/node_modules/sweet.js/lib/sweet.js:1874:20)

[disnet]

Ah! Looks like I led you down the wrong path. We don't currently expand macros in the parameter position of a function, and I don't think we ever will but need to think about that more. Clearly we need a better error message though.

You might think we could overcome this by using another helper macro, something like:

macro expand_params {
    case ($param ...) => {
        $param ...
    }
}

macro helper_def {
  case ($params ...) { $body ... } => {
    (function ($params ...) { $body ... })
  }
}

macro def {
    case ($params ...) { $body ... } => {
      helper_def (expand_params($params ...)) { $body ... }
    }
}

def(a, b) {
    return a + b;
}

And this should work but a bug is preventing nested macros from being expanded (I'm currently working on fixing this).

[eknkc]

Hey, thanks for the info. I actually tried this one too, facing some error, backed up to former one.

BTW, this is the sole "compile to javascript" project that makes a lot of sense to me. Great work, hope it matures as fast as possible, waiting to use it in production :)

[disnet]

Great the hear! Thanks for being willing to bang your head against it for a little bit :)