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Re: A bug?

Miki Tebeka wrote:
> 
> Hello All,
> 
> It this a bug:
> 
> > (gensym)
> g47
> > (define g48 'foo)
> > (gensym)
> g48
> 
> (This is mzscheme version 101)
> 
> Or gensym does something else than I think?

Gensym generates *uninterned* symbols.  That means that the "g47" that
was generated the first time is distinct from any other symbol called
"g47."  Ditto for g48.

So, look at the following:
> (define a (gensym))
> a
g4
> (define b 'g4)
> a
g4
> b
g4
> (eq? a b)
#f
> (equal? a b)
#f

A main use of gensym is to generate symbols that can never be generated
again.  

This is particularly useful for building names/identifiers in macros;
the goal is to have a name that you know cannot conflict with anything
that an application might generate.

I used it in a set of R5RS functions for manipulating
"structures/records;" I wound up using gensym to generate a "signature"
to indicate that a particular structure instance was indeed generated by
the structure system.
-- 
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