Re: Semantics of circular imports

[Maxime Devos <maximedevos@telenet.be>]

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Philip McGrath schreef op zo 27-03-2022 om 10:12 [-0400]:
> In the context of Racket or R6RS modules, where the semantics are 
> essentially those of `letrec*`, I'm used to distinguishing "unbound"
> variables from "undefined" variables, two types of errors, though
> informally "defined" is often used more loosely: 

Guile also distinguished them, though possibly in a different way, and
terminology is inconsistent.

> 
>   1. Every variable reference must refer to a lexically visible
>      binding, or it is an "unbound" variable error. Inherently, this
>      can *always* be detected statically at compile time.

This is different in Guile.  If a variable reference 'foo' is not a
lexical variable, then Guile assumes it is a global variable -- it
doesn't check if the name of the global variable reference actually
belongs to an imported module, this only happens at runtime. 
Apparently Guile is a bit lazier than Racket.

> 
>   2. A variable may not be accessed until it has been initialized.(
>      More precisely, we could discuss a variable's "location", which
>      I see is part of Guile's model.) This is the essence of the
>      `letrec`/`letrec*` restriction: while the "fixing letrec"
>      papers[4][5] show that useful static analysis can be done, this
>      is fundamentally a dynamic property. Violating the restriction
>      is an "undefined" variable error at runtime.

I don't know how Guile handles letrec/letrec*.  However, Guile has
undefined(uninitialised? unbound? Guile has inconsistent terminology
here) variables:

(define foo) ; no value
foo ; I thought this would raise an "Unbound variable: ..." error but apparently not?  A bug? Looks like it returns *unspecified* instead ...

(variable-ref (make-undefined-variable)) ; -> Unbound variable: ...

(define f (make-variable #f))
(variable-unset! f)
(variable-ref f) ; --> Unbound variable: #<variable 7fb531f1cce0 value: #<undefined>>

(variable-ref (make-variable #f)) ; this variable has no name, so in a sense, it is not ‘bound’, yet it has a value

Greetings,
Maxime.

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