Hi there,
I'm trying to get a grip on macros. Here is a very simple Racket
expression (1):
(letrec [(a 2)
(b 3)
(afn (lambda (x) (+ x a)))
(bfn (lambda (x) (+ x b)))]
(afn (bfn 2)))
Now I need a syntactic abstraction for afn and bfn. The following will
do in first approximation (2):
(define-syntax-rule (absfn varname) (lambda (x) (+ x varname)))
(letrec [(a 2)
(b 3)
(afn (absfn a))
(bfn (absfn b))]
(afn (bfn 2)))
However, it will fail due to scoping rules in the following example (3):
(define-syntax-rule (absfn varname) (lambda (x) (+ (+ x localc) varname)))
(letrec [(a 2)
(b 3)
(localc 4)
(afn (absfn a))
(bfn (absfn b))]
(afn (bfn 2)))
In other words, my syntactic extension absfn needs to be embedded in
the namespace of the sorrounding expression (or as a "dumb" macro
which simply does lexical replacement without considering scoping, but
needless to say such a macro would be unhygienic).
I suspect that letrec-syntax was meant for that purpose, but I can't
figure out how the parameters to define-syntax-rule would translate to
those of letrec-syntax.
Does anyone have sample code for how to get (3) above to work?
Thanks!
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