Re: [Scheme-reports] library syntax: "visiting" a library left undefined?

On Fri, Jan 4, 2013 at 8:39 AM, John Cowan <cowan@x> wrote:

Aaron W. Hsu scripsit:

> 1) Evaluation of the library forms, but not the arbitrary code bodies of
> those forms, giving you information on the imports and exports of the
> library, and gives a specific code body for the following steps, based
> on the results of cond-expand;

Thanks, this is very helpful. IIUC, I would say "examination" rather than
"evaluation" here, as cond-expanding is not really evaluation.

> 2) Visitation, which gives you definitions of the keywords in the
> library, but does not evaluate any of the code. This is akin in some
> systems to expansion; and finally,
>
> 3) Instantiation, which evaluates the code definitions and binds their
> exports, as well as evaluating the forms in the body that are found.

Okay. I have changed the wording again to this:

Similarly, during the expansion of a library {\cf foo}, if a syntax
keyword imported from another library {\cf (bar)} is needed to

expand the library, then the syntax definitions of {\cf (bar)}
must be loaded and expanded before the expansion of {\cf (foo)}.

Loading is not necessary, as Aaron pointed out.

In our simplified situation with no low-level macros, we

really only need to expand the library, and in fact

we don't have to expand lambda bodies, so the minimum

requirement is a superficial trace of the library skeleton,

"evaluating" top-level macros. This trace is what's generally

referred to as visiting, and may simply be expansion.

Although Aaron is correct that in the general case we

can't single out the specific macros used, it is not

impossible in all cases, and would therefore be wrong

for the standard to forbid such optimizations.

Note also the timing is unspecified. Chicken uses a batch

compiler to separate syntax definitions from normal

definitions, and load only the syntax on import (but loads

all syntax, not restricting to just the used forms). In

this case the visit has already been done once, in a

sense, but is not re-performed every time. I guess a

general "before" covers this, since we say nothing of

how long before or how many times.

Assuming we don't want to add a long discussion of the

definition of visitation for something that is fairly simple

in the small language, I would suggest:

Similarly, during the expansion of a library {\cf foo}, if

any syntax keywords imported from another library {\cf (bar)}

are needed to expand the library, then the corresponding syntax

definitions of {\cf (bar)} must be expanded before the

expansion of {\cf (foo)}.

Alex