[r6rs-discuss] [Formal] Querying Declaration Quality Priorities

[MichaelL at frogware.com]

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Name: Michael Lenaghan
Email: michaell at frogware.com
Type: Enhancement
Priority: Major
Component: Declarations
Version: 5.91
Section: 9.22 (pg 56)
Dependencies: None

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.