RE: [Jscheme-user] a little problem with define-macro

[Borislav I. <bor...@ko...>]

Hi Tim,

Thanks very much for the explanation. However, I must admit that the
workings of those macros are a bit confusing to me. I can stupidly
remember to unquote the macro arguments in a returned s-expression, but
I don't quite understand why and how it works. 

Ok, define-macro returns an s-expression which will _then_ be evaluated.
So when a macro is applied, the s-expression is first constructed from
the macro definition and then it is passed to the evaluator. So with the
following macro:

(define-macro (my-delay form) `(delay ,form))

If I write:

(define promise (my-delay (display "Hi there!")))

what is happening is that first an s-expression is constructed:

`(delay ,(display "Hi there!"))

which is then evaluated to yield the promise. So the 'promise' define
above should be equivalent to:

(define promise (eval `(delay ,(display "Hi there!"))))

But evaluating the above expression prints the string "Hi there!" as one
would expect because (display ...) is unquoted inside the
quasiquotation. 

Evaluating (my-delay (display "Hi there!")) doesn't display anything
which is the intent, but I don't understand why? Given the implied model
of macro expansion (i.e. build an s-expression, then evaluate it).

Thanks,
Boris

| -----Original Message-----
| From: Timothy John Hickey [mailto:tim...@ma...] 
| Sent: Wednesday, April 07, 2004 9:38 AM
| To: Borislav Iordanov
| Cc: jsc...@li...
| Subject: Re: [Jscheme-user] a little problem with define-macro
| 
| 
| 
| On Apr 7, 2004, at 12:27 AM, Borislav Iordanov wrote:
| 
| > Hi,
| >
| > I tried using define-macro (i.e. the built in macros) to simplify 
| > working with streams thus:
| >
| >> (define-macro (make-stream h t) (cons h (delay t)))
| > (macro make-stream (h t)...)
| 
| The define-macro should return an s-expression which will
| then be evaluated:
| 
|      (define-macro (make-stream h t) `(cons ,h (delay ,t)))
| 
| This works fine with enum. The quasiquoted expression
| could also be written with standard quote as
| 
|    (define-macro (make-stream h t) (list 'cons h (list 'delay t)))
| 
| which would work as well, but is harder to read...
| 
| ---Tim---
| 
| 
| >
| > But then:
| >
| >> (define (enum start end)
| >    (if (> start end) '() (make-stream start (enum (+ start 
| 1) end))))
| >
| > SchemeException: expected object of type list (i.e. pair or empty), 
| > but
| > got: , "
| > (lambda delay~3 ()...)"
| >         at jsint.E.error(E.java:14)
| >         at jsint.E.typeError(E.java:24)
| >         at jsint.U.toList(U.java:180)
| >         at jsint.U.listToVector(U.java:772)
| >         at jsint.Scheme.analyze(Scheme.java:487)
| >         at jsint.Scheme.analyze(Scheme.java:475)
| >         at jsint.Scheme.analyze(Scheme.java:490)
| >         at jsint.Scheme.analyze(Scheme.java:473)
| >         at jsint.Scheme.analyze(Scheme.java:459)
| >         at jsint.Scheme.analyze(Scheme.java:490)
| >         at jsint.Scheme.analyze(Scheme.java:490)
| >         at jsint.Scheme.analyze(Scheme.java:475)
| >         at jsint.Scheme.eval(Scheme.java:434)
| >         at jsint.Scheme.eval(Scheme.java:424)
| >         at jsint.Scheme.readEvalWriteLoop(Scheme.java:208)
| >         at jsint.Scheme.runJscheme(Scheme.java:186)
| >         at jsint.Scheme.defaultMain(Scheme.java:149)
| >         at jsint.Scheme.main(Scheme.java:124)
| >         at jscheme.REPL.main(REPL.java:138)
| >
| >
| > If expand "manually" the macro:
| >
| > (define (enum start end)
| >    (if (> start end) '() (cons start (delay (enum (+ start 
| 1) end)))))
| >
| > there is no problem.
| >
| > BTW, it also works with define-syntax macros..
| >
| > Best,
| > Boris
| >
| >
| >
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