Hello,
Suppose I'd like a type-checking macro that looks something like this:
(define-syntax (type-check stx)
(syntax-case stx ()
[(_ (+ x y)) (if (and (number? (type-check #'x))
(number? (type-check #'y)))
#'Number #'(error "bad types!"))]
[*cases for other expressions]))
The macro will examine the syntax it's given and determine the type that will
be produced at runtime.
The key here is that type-check needs to call itself, but that's not possible.
Type-check is defined to be used in phase 0, but the recursive call to
type-check takes place in phase 1. If the recursive call to type-check makes a
recursive-call, then this second call would be running in phase 2 and so on.
The compiler won't allow me to use this macro in a phase it's not defined for,
so this code is no good.
So, first question: How can I get a macro to call itself recursively such as in
this case? I can't wrap the entire if in a syntax object, because I want to
throw that error in phase 1, so the condition of the if statement needs to be
evaluated inside this macro.
Second, even if I get type-check to work in all phases, if I call type-check
with (+ 1 2), the recursive call to (type-check #'x) will get #'x as its
"argument" instead of 1, no?
It almost feels like I should be doing this with a regular old function that
takes in syntax, and produces a type, but to my knowledge, I can't use ... in
runtime expressions such as the following
(define (type-check stx)
(syntax-case stx ()
[(body) *dummy-value*]
[(body ...) (begin (type-check #'body) ...)]))
This will result in a syntax-error telling me that ... can't be used as an
expression like this, which makes it impossible to deal with ... forms.
Any advice?
Thanks in advance.
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