Re: [racket-users] Context aware macro

[Sorawee Porncharoenwase]

>
> But what surprises me the most is that it only works at the top level:
> assign.rkt>
> assign.rkt> (assign foo 3)
> assign.rkt> (assign (bar x) (assign foo 7) (* x foo))
> assign.rkt> (bar 1)
> 7
> assign.rkt> global
> '#hash((bar . (λ (x) (assign foo 7) (* x foo))) (foo . 3))
>
Interesting. Following your steps, I got a different result that makes more
sense:

'#hash((bar . (λ (x) (assign foo 7) (* x foo))) (foo . 7))

Another question: my macro doesn't work with (f . xs) notation. Of course,
> I could add a [_ f . xs] syntax-case. But I would also have to add a [_ f a
> ... . xs]. Plus keywords. Is there a way not to have to describe every
> possible situation?
>
For a “flat” function definition, (_ f a ... . xs) is the most general
form. It encompasses patterns like (_ f . xs), where a matches 0 times, and (_
f 1 2), where xs is (). So if you have that, you don’t need any other cases.

FYI: function definition allows the curry form too. E.g.,

(define ((f x) y)
  (+ x y))

The most convenient way to parse the header is to switch from syntax-case
to syntax-parse, and use the function-header syntax class in
syntax/parse/lib/function-header. Lambda doesn’t support currying notation
though, so maybe you don’t care about it.

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> 
> .
>

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[racket-users] Context aware macro

[Dimaugh Silvestris]

I wanted a 'define' that stores in a hash the arguments and the function
definition so I don't keep forgetting argument order and having to go to
the file and check. So with my very little knowledge of macros, I wrote
this:

#lang racket

(define global (make-hash))

(define-syntax (assign stx)
  (syntax-case stx []
([_ (f xs ...) b ...]
 #'[begin
 (hash-set! global 'f (list 'λ '(xs ...) 'b ...))
 (define (f xs ...) b ...)])
([_ a b]
 #'[begin
 (hash-set! global 'a b)
 (define a b)])))

Surprisingly it works:
assign.rkt> (assign (sum a b) (+ a b))
assign.rkt> global
'#hash((sum . (λ (a b) (+ a b

But what surprises me the most is that it only works at the top level:
assign.rkt>
assign.rkt> (assign foo 3)
assign.rkt> (assign (bar x) (assign foo 7) (* x foo))
assign.rkt> (bar 1)
7
assign.rkt> global
'#hash((bar . (λ (x) (assign foo 7) (* x foo))) (foo . 3))

I suspected I would have had some trouble with local definitions colliding
with the global hash, but luckily it isn't happening. Why is that?

Another question: my macro doesn't work with (f . xs) notation. Of course,
I could add a [_ f . xs] syntax-case. But I would also have to add a [_ f a
... . xs]. Plus keywords. Is there a way not to have to describe every
possible situation?

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