Re: [Chicken-users] unbound variable: or

[John Cowan]

chi scripsit:

> But I can't find anywhere that really explains why
> that's _fundamentally_ broken, not just ugly.

Define-macro has two problems: it prevents (as opposed to selectively
escaping from) hygiene, and it only supports global macros.
Unhygienic syntax has exactly the same problem as dynamically scoped
variables in Elisp or other pre-Common Lisps: you can't tell what
code does without examining its use of identifiers.

> Incidentally, I'm pretty sure define-syntax isn't hygenic. syntax-rules is
> hygenic. You can still ruin yourself with define-syntax, like:

That's true.  Low-level macro systems allow both hygiene and non-hygiene,
and both can be defined using define-syntax, let-syntax, let*-syntax,
or letrec-syntax.

> (define-syntax a
>   (lambda (form rename compare)
>     '(define b 23)))

For portability, you should always wrap the lambda in (er-macro-transformer 
...),
even though that does nothing on Chicken.  That way, if your code is ever
ported to a Scheme that provides syntax-case, you'll get an immediate
error rather than randomly going wrong.

> I would like to state for the record that even defining hygenic syntax is
> a pretty ruinous idea. You have to be very careful of what you're doing, and
> confident that it's a good idea, because it is literally impossible for 
> someone
> to tell what your program means without first calculating all of the syntax
> rules you have defined in their head. 

That's where syntax-rules shines: it gives up the full power of Scheme
in favor of safety and clarity.  IMO, there is really no need to use
low-level macros ever.

> It is kind of fun that you can write an uncomputable program though.

It makes things like continuous build servers hard, though.

-- 
John Cowan          http://www.ccil.org/~cowan        address@hidden
Mos Eisley spaceport.  You will never see a more wretched hive of scum
and villainy --unless you watch the Jerry Springer Show.
        --georgettesworld.com