[r6rs-discuss] [Formal] Remove double phase semantics

From: William D Clinger <will>
Date: Sat Nov 25 19:16:18 2006

I am posting this as an individual member of the Scheme
community. I am not speaking for the R6RS editors, and
this message should not be confused with the editors'
eventual formal response.

AndrevanTonder wrote:

> The much cleaner semantics is the one in which a separate set of
> bindings is instantiated for each level.

Marcin 'Qrczak' Kowalczyk questioned whether that semantics
works for this use case:

> Let's imagine a Scheme compiler which hosts its own interpreter.
> Certain libraries (e.g. the standard library) are provided only in the
> compiled form, and the interpreter just links to their version which
> is compiled into the interpreter program instead of interpreting their
> source. Assume that they contain mutable state.

Here's another use case that seems more likely to work with
shared bindings than with the allegedly cleaner semantics:

Suppose string->symbol is implemented as a library, using a
hash table that maps strings to symbols as its mutable state.
With shared bindings, I think procedural macros would see the
same symbols that are seen at run-time. With the allegedly
cleaner semantics, the symbols that would be available to
procedural macros at macro expansion time would be completely
different from the symbols that exist at run time.

Maybe that doesn't matter; I don't see anything in the draft
R6RS that would require the symbols seen at macro expansion
time to have anything to do with the symbols seen at run
time. If the R6RS were to require symbols to be the same
at run time and at macro expansion time, however, then the
R6RS ought not to require a semantics that makes it difficult
or impossible to implement string->symbol in Scheme.

Will
Received on Sat Nov 25 2006 - 19:16:10 UTC