[r6rs-discuss] [Formal] Phase semantics

[AndrevanTonder]

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Name        : Andre van Tonder
Email       : andre at het.brown.edu
Type        : defect
Priority    : minor
Component   : libraries
Version     : 5.91
Pages       : 23
Dependencies: None

Summary:
--------

Slight refinement in specification of phase semantics for better portability.

Description:
------------

At the beginning of section 6.2, the sentence

   "An exception may be raised, however, if a binding is used out of its declared
    phase(s)"

should preferably be changed to

   "An exception [must] be raised, however, if a binding is used out of its
    declared phase(s)"

As far as I am aware, all three existing reference implementations do this, 
so it should be no problem.  Leaving it unspecified can only hinder portability.

On page 24, the sub-paragraph:

   "... Similarly, an implementation is allowed
    to distinguish invocations of a library across different
    phases or to treat an invocation at any phase as an invocation
    at all phases.  An implementation
    is further allowed
    to start each expansion of a library form by removing all
    library bindings above phase 0.  Thus, a portable library's
    meaning must not depend on whether the invocations are
    distinguished or preserved across phases or library expansions."

should preferably be

   "... Similarly, an implementation is allowed
    to distinguish invocations of a library across different
    phases or to treat an invocation at any phase as an invocation
    at all phases.  [no changes]

    [changed position of this to avoid confusion:]
    Thus, a portable library's
    meaning must not depend on whether the invocations are
    distinguished or preserved across phases or library expansions.

    An implementation
    is further [required]
    to start each expansion of a library form by removing all
    library bindings above phase 0."

Again, as far as I am aware, all three existing reference implementations 
do this, so again it should be no problem.  Leaving it unspecified can only
hinder portability.

Furthermore, it may be helpful to point out that in the following:

   "An implementation is allowed to distinguish visits of a library
    across different phases or to treat a visit at any phase
    as a visit at all phases"

the first kind of implementation may (or perhaps must?) use a separate space of 
bindings for each phase, whereas in the second case the bindings are shared 
between phases, but are only available to be used in the phases stated in the 
"for ---" specifier, and that portable libraries should not rely on either
semantics.  This can be a highly confusing issue for the initiated and 
uninitiated alike, and a little more explanation could be very useful.