[plt-scheme] probably dumb question, but...

  ,a

is, at read time, expanded into:

  (unquote a)

They are identical by the you get to the macro expander, etc. Note that
reader macros aren't expanded inside ""s or inside ||s.

So, it may become clearer if you go back thru your examples and
substitute (unquote a) for everything.

After that, try this one:

  (define funny
    (lambda ,a
      (let (,a) ,a)))
  
  (funny funny values)
    

Moral: yes, Scheme can be as wacky (for no good reason) as any other
language.

Robby

At Mon, 03 Jan 2005 14:33:44 -0500, Psy-Kosh wrote:
>   For list-related administrative tasks:
>   http://list.cs.brown.edu/mailman/listinfo/plt-scheme
> 
> I was messing around a bit, and discovered something... odd.
> 
> consider the following:
> 
> Welcome to DrScheme, version 209.
> Language: Textual (MzScheme, includes R5RS).
>  > ;this is expected:
>  > ,a
> unquote: not in quasiquote in: (unquote a)
>  > ;the rest of this confuses me:
>  > ',a
> ,a
>  > (symbol? ',a)
> #f
>  > (eq? ',a ',a)
> #f
>  > (eqv? ',a ',a)
> #f
>  > (equal? ',a ',a)
> #t
>  > ;hrm, but even though it's not a symbol...
>  > (define ,a 37)
>  > ,a
> . reference to undefined identifier: a
>  > a
> . reference to undefined identifier: a
>  > |,a|
> . reference to undefined identifier: |,a|
>  > ;so ,a doesn't produce an unquote not in quasiquote error anymore, 
> but there doesn't seem to be any way to extract the 37 from wherever it is
>  > (set ,a 26)
> . reference to undefined identifier: set
>  > ;typo
>  > (set! ,a 26)
> set!: not an identifier in: (unquote a)
>  > ;but...
>  > (define ,a 26)
>  > ;but... but... aren't later defines defined to act equivalently to set! ?
>  >
> 
> okay, so what exactly is ,a? why if it's not a symbol, why can it be 
> used as an identifier for the purposes of define? why not for set! ?
> 
> Psy-Kosh