Re: Semantics of circular imports

[Zhu Zihao]

Philip McGrath <philip@philipmcgrath.com> writes:

> Oh, wow. I definitely had not realized that, *even inside a declarative 
> module*, a reference to a variable with no statically visible definition 
> would semantically be a dynamic lookup in a mutable environment at 
> runtime (rather than a compile-time error), though I do see now that 
> `info guile declarative` does indeed say that marking a module as 
> declarative "applies only to the subset of top-level definitions that 
> are themselves declarative: those that are defined within the 
> compilation unit, and not assigned (‘set!’) or redefined within the 
> compilation unit."

It depends on how you reference to it. If a variable is referenced in
the same module, it'll be inlined if it's small enough.

If you reference it from another module, it'll do something like
module-ref instead (works like non-declarative binding)

> This seems like a big barrier to cross-module inlining, though IIUC Guile
> currently doesn't do much of that by default (maybe for this reason).

Guile 3.0.8 comes to rescue. It works by rewrite the `define-module`
call and attach the tree-il of some bindings on it.

For details, read the blog post authored by Andy Wingo

https://wingolog.org/archives/2021/05/13/cross-module-inlining-in-guile

TIPS: If you're interested in the implementation detail of Guile. You
should not miss the Andy Wingo's blog post!  

> The use of "top-level" to refer to definitions within a module is 
> somewhat confusing to me. I usually understand "top-level" to refer to 
> the kind of interactive REPL environment for which R6RS leaves the 
> semantics unspecified. Racket uses "module-level variable"  and "module 
> context" in contrast to "top-level variable" and "top-level context" to
> make this distinction.[1][2][3] (There are also R6RS "top-level 
> programs", but I wouldn't think of those unless made very clear from 
> context.)

Top-level is the set of outermost brackets in a file :)

The optimization pass related to the top-level in Guile is called
letrectify. It rewrites the top-level bindings into a big letrec block.
You can check the comment of (language tree-il letrectify) and it has
a good example.

Because top-levels can reference each other, so the evaluation order
will not the same as the code order.

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Zihao

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