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Re: Opaque syntax objects



Andre van Tonder wrote:
On Sat, 13 Aug 2005, [ISO-8859-1] Jens Axel Søgaard wrote:

Representing syntax-objects using normal lists does breaks the
abstraction, and I too prefer the extra layer of abstraction.

This is often said, but I've never understood why people would think this. Normally subtyping (see my earlier message) or genericity are regarded as abstraction mechanisms, not abstraction-breaking mechanisms. After all, people don't normally say that generic + and * break the "integer" abstraction. Why should generic car/cdr be different here?

This is largely a matter of taste, but I don't think of syntax as lists,
where as I think of integers as numbers.

Keeping this abstraction doesn't make destructuring significantly more
impractical. Destructuring arguments using the pattern matching
capabilities of syntax-case is unchanged with both representations,

I feel strongly that pattern matching should be library syntax. For this proposal, I personally prefer a design based on a small procedural core + [quasi]syntax, rather than a rather complex special form.

The purpose of the do-primes example was to show that simple destructuring is possible without using syntax-case.

and
in the case were it is neccessary to use normal Scheme operators, most
often a call to syntax->list, which turns a syntax-object representing a
list into a list of syntax-objects, is enough to solve the problem.

This can entail a rather expensive performance hit.

What is "rather expensive"?

--
Jens Axel Søgaard