Formal comment #77 (enhancement)

Querying Declaration Quality Priorities
Reported by:	Michael Lenaghan

Component: Declarations 

Section: 9.22 (pg 56) 

Summary

Developers should have access to declaration quality priorities so
that developer code can take the current value into account.

Description

Chez Scheme has an optimize-level parameter. The value of
optimize-level can be both read and written. Currently, optimize-level
3 means that the compiler is allowed to generate unsafe
code. Developers can query the value of optimize-level at expansion
time so that expanded code takes the current setting into account.

For example, in Chez Scheme it's possible to write code like this:

(define-syntax define-something
    (lambda (e)
        (syntax-case e ()
            [(_ (field ...))
                (with-syntax ([no-type-check (>= (optimize-level) 3)])
                    #'(<some code>
                            (if (or no-type-check (check-type? <some code>))
                                <some code>
                                <some error code>)
                        <some code>)])))

Chez Scheme itself uses an approach like this to avoid record type
checks in record field accessors when expanding records at
optimize-level 3.

To write code like this--especially in a portable way--developers need
access to the current values of the declaration qualities.

RESPONSE:

In response to a different formal comment, declarations
will be dropped from the report.

The second sentence of section 4.4 will be revised to say
something like "Libraries and scripts that import only
from safe libraries are also said to be safe."  The last
sentence of section 4.4 will be revised to something like
"Implementations may provide access to unsafe libraries,
and may provide other implementation-specific facilities
that cannot guarantee safety."

Implementation-specific facilities may also be mentioned
in a non-normative appendix.

The report will abandon the second half of the guiding
principle stated in the R6RS Status Report of June 21,
2006, that R6RS Scheme should:

  * allow programmers to rely on a level of automatic run-time
    type and bounds checking sufficient to ensure type safety
    while also providing a standard way to declare whether such
    checks are desired;