[r6rs-discuss] Implicitly Concurrent Scheme
cowan at ccil.org
Thu Nov 12 16:40:35 EST 2009
Aubrey Jaffer scripsit:
> I am suggesting the following restriction be *removed* from the
> "Procedure calls" section of RnRS:
> _Note:_ Although the order of evaluation is otherwise unspecified,
> the effect of any concurrent evaluation of the operator and
> operand expressions is constrained to be consistent with some
> sequential order of evaluation. The order of evaluation may be
> chosen differently for each procedure call.
> Removing a restriction would seem to be justified solely on the basis
> Programming languages should be designed not by piling feature on
> top of feature, but by removing the weaknesses and restrictions
> that make additional features appear necessary.
This is a fine example of the rhetorical fallacy of equivocation.
The second quotation refers to removing restrictions on programmers.
The first quote imposes a restriction on implementations, partly for
the benefit of those same implementations, but mostly for the benefit
of programmers trying to reason about side effects.
R5RS has a restriction that the forms in a BEGIN must be evaluated
sequentially and in order. Lifting this restriction would benefit some
uses, but would force programmers to devise a new way of doing things
sequentially and in order (like output), because they could no longer
rely on BEGIN.
Like all contracts, a spec specifies the agreements between implementers
and users. Each accepts some restrictions.
Using RELAX NG compact syntax to John Cowan <cowan at ccil.org>
develop schemas is one of the simple http://www.ccil.org/~cowan
pleasures in life....
--Jeni Tennison <cowan at ccil.org>
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