Re: Recursive Macros generating Definitions

[Maxime Devos]

On 03-10-2022 13:32, Frank Terbeck wrote:
> When looking at  this, I also saw the following,  which might be related
> if ‘syntax-rules’ is implemented using  ‘syntax-case’

It is, IIRC.

> (I didn't check if
> this is the case):
>
>      (define-syntax-rule (foobar n) (define quux n))
>      ,exp (foobar 23)
>    → (define quux-ea7bdcf8675f4a4 23)

This is correct (as in, functioning as intended and not a bug) to my 
understanding -- in the match expression of 'foobar', 'quux' does not 
appear, so the for hygiene, the 'quux' inside shouldn't be the quux outside.

Compare:

(define-syntax-rule (define-pair-contents pair the-car the-cdr)
  (begin
    (define p pair) ; only compute it once.  Due to lexical hygiene, 
this won't interfere with any 'p' in the environment.
    (define the-car (car pair))
    (define the-cdr (cdr pair)))).

-- this shouldn't be expanded to

(define p pair)
(define the-car (car p))
(define the-cdr (cdr p))

because of hygiene (the environment might already be using 'p' for 
something else).

It's sometimes a bit inconvenient -- sometimes you _want_ to define 
'quux' (and not just only available to the macro), but that's easily 
resolved by adding an additional 'quux' argument to 'foobar':

      (define-syntax-rule (foobar quux n) (define quux n))
      ,exp (foobar quux 23)

> (define-syntax generate-shorthands [...]

Your recursive macro is, well, recursive.  This is fine, but IIUC a 
consequence of this is that the recursive 'call' to generate-shorthands 
is a new lexical lexical environment (hence, hygience, so -?????? stuff).

As such, I consider this not a bug in Guile, but a bug in your code.

My proposal would be to change the 'x' in (datum->syntax x) -- instead 
of using #'x (which refers to the whole expression, which in a recursive 
call has an undesired lexical environment), use something of the 
'end-user' of generate-shorthands, say, #'s (i.e., SEMANTICS-SYMBOL) 
(for the right lexical environment).

If I make that change, I get some reasonable output (no -????? suffixes):

$1 = (begin
  (define (varint:sint32-decode bv)
    (varint-decode bv 32 zig-zag))
  (define (varint:sint32-encode n)
    (varint-encode n 32 zig-zag)))

Greetings,
Maxime.

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