[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Rest and patterns



Neil W. Van Dyke wrote:
Although, as devil's advocate, I'd point out that the "lambda" syntax
has N-terms-plus-rest as one pattern language.  So, if "let" uses
N-terms-plus-rest and nothing else, there is at least precedent.

But there's no precedent for the `rest' syntax in lambda. And Al Petrofsky pointed out an issue with using the lambda semi-variable-arity feature.

My concerns with that would be: ...

I agree with your concerns about making this a general pattern matching SRFI; such a SRFI would be quite an undertaking, and it doesn't help matters that there is no way to define a datatype in R5RS. (Luckily there is SRFI 57).

André van Tonder has been working on a pattern matching proposal for some time, and I've tried to contribute to it. In that proposal the language of patterns is extensible. The way `match' works is it examines the head of the pattern. If it is a keyword associated with a pattern rewrite rule (match implicitly carries around an environment binding keywords to syntax transformations), then the match expression is transformed accordingly, otherwise it's interpreted as a regular variable. Using the `values' keyword in `let' would work seamlessly with this approach to pattern matching.

Or, on the other hand, perhaps this SRFI should remain within it's
current scope, but in that case I would suggest leaving the extension
of `let' in such a way that it is still possible to extend `let' to
use patterns in the future.  For this reason, I suggest keeping the
`values' keyword that Neil W. Van Dyke recently argued against.

I don't immediately see how this "values" syntax would be a natural part
of the future TBD pattern language, nor how the "(rest VAR)" syntax
precludes a future TBD pattern language.  If you are asserting one of
those, could you say a bit more about that?

I think my point was having a keyword at the head of the form is helpful, but in particular whenever there are keywords within the form whose meaning depends on the context. Adding a `rest' keyword to `let' seems like a hack, but adding a `rest' keyword to a `values' pattern seems appropriate.

But really all I wanted to do was point out that we're now talking about pattern matching, even if only for multiple values.

David