[Larceny-users] cyclic library dependency

[Alex Shinn]

Thanks for the explanation Will, that makes sense.

It seems from the rationale that at least some authors
considered the possibility of cyclic libraries.  That
combined with the loose, phase-free specification of
R7RS-small libraries makes me think we did the right
thing (though personally I'm strongly opposed to cyclic
dependencies).

R7RS-large will include low-level macros and by
necessity a specification of semantics regarding
phasing, and should therefore address this directly.

-- 
Alex

On Sun, Jan 31, 2016 at 11:31 PM, William D Clinger <will at ccs.neu.edu>
wrote:

> Alex Shinn wrote:
>
> > Where does R6RS state it forbids cyclic libraries?
>
> That's a consequence of R6RS Section 7.3, and is stated
> a bit more directly in the (never-ratified) R6RS Rationale
> Section 7.
>
> What I'm about to quote is easier to understand if you recall
> that, in the absence of macros, all variables are referenced
> at phase 0.  From R6RS Section 7.3:
>
>     If any of a library’s definitions are referenced at
>     phase 0 in the expanded form of a program, then an
>     instance of the referenced library is created for
>     phase 0 before the program’s definitions and
>     expressions are evaluated. This rule applies
>     transitively: if the expanded form of one library
>     references at phase 0 an identifier from another
>     library, then before the referencing library is
>     instantiated at phase n, the referenced library
>     must be instantiated at phase n.
>
> Note the word "must" in that last sentence.  Note also the
> two uses of the word "before".  If some library B imports
> another library A, then the above says library A "must" be
> instantiated "before" library B is instantiated.
>
> A library can't be instantiated at level 0 before it is
> instantiated at level 0, so cyclic library dependencies
> are forbidden by the R6RS absolute requirement ("must")
> quoted above.
>
> The R6RS Rationale Section 7 says:
>
>     The library system does not address the following
>     goals, which were considered during the design
>     process:
>
>       * independent compilation
>       * mutually dependent libraries
>       * separation of library interface from library
>         implementation
>       * local modules and local imports
>
> The only one of those four that is clearly forbidden by
> an absolute requirement of the R6RS is the second one:
> mutually dependent libraries.
>
> Will
>
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