[plt-scheme] What are syntax-case pattern variables?

Grant Rettke wrote:
> Hi folks,
> 
> syntax-case takes a syntax object as an argument and destructures it
> via pattern matching into pattern variables.
> 
> What are pattern variables composed of?
> 
> syntax takes a template, and the pattern variables in it, to produce a
> syntax object that has the same lexical scoping as that of the pattern
> variable, so where does that lexical scope get stored?

The short answer is that syntax objects store scoping information and 
that pattern variables have nothing to do with it.

Here's a longer answer:

A pattern variable is essentially bound to an ellipsis-depth n and 
syntax object(s). If the pattern variable doesn't have ellipses after 
it, then n is 0 and the pattern variable refers to a single syntax 
object. If it has one ellipsis after it, then n is 1 and there is a list 
of syntax objects. And so forth.

The 'syntax' form is like 'quasiquote', except that there is no explicit 
'unquote'; rather, pattern variables are automatically substituted, and 
the rest of the template becomes syntax object literals (via 
'quote-syntax') that remember the bindings in scope where they occurred.

So pattern variables, while convenient, are not necessary for writing 
macros. You could do just the same with 'quote-syntax', 'syntax-e', 
'datum->syntax', 'car', etc.

Lexical scoping is managed by the syntax objects. The macro arguments 
already have their lexical contexts embedded in them, and that is 
preserved as the macro takes them apart and puts them together with new 
syntax. This new syntax mostly comes from syntax literals.

The essence of the algorithm is explained in "Syntactic Abstraction in 
Scheme" by Dybvig et al.

Ryan


> 
> Best wishes,
> 
> Grant Rettke
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