Re: [Kawa-commonlisp-dev] Appropiate expression class

[Per Bothner]

On 03/03/2013 11:08 AM, Charles Turner wrote:
> Hi all,
>
> For the default values in ordinarly lambda lists, I can't see which
> Expression class should properly hold them.

The general class is Expression.  Commonly it's a QuoteExp, but not always,
but the default value is an arbitrary expression.

(I think you've been away from Kawa too long, since I believe
you know this if you think about it ...)

Kawa handles this for Scheme.  The only extra complication for
Common Lisp is the supplied_p variable.

One way to handle supplied_p without changing gnu.expr is with a
dummy variable:

(lambda (&optional (x x_default x-p)) body)

becomes:

(lambda (&optional ($incoming$x #!undefined))
   (let ((x_p (eq $incoming$x #!undefined))
         (x (if x_p $incoming$x x_default)))
     body)

> My first thought was a
> QuoteExp, so to generate the abstract syntax for checking a whether an
> argument was passed, I had something like:
>
> new IfExp(new ReferenceExp(supplied_p variable),
>              new ReferenceExp(variable passed in),
>              new QuoteExp(default value));
>                     ^^^^^^^^^
>
> The QuoteExp here isn't right, because with I get results like this:
>
> #|kawa:2|# ((lambda (&optional (x 'a x-p) (y 'b y-p) (z 'c z-p)) (list
> x y z x-p y-p z-p)))
> ((quote a) (quote b) (quote c) () () ())
>
> Here, because the default is of the form (quote x), I'm creating
> (quote (quote x)). I'm not sure which Expression class best represents
> this "any valid lisp form".

The Translator#rewrite methods take care of this.
--
        --Per Bothner
address@hidden   http://per.bothner.com/