Re: Dynamic variable binding

[Keith Wright]

> From: Sebastian Tennant <address@hidden>
>
>  guile> (define-macro (definer var val)
>           `(define ,var ,val))
>  guile> (definer 'foo "bar")
>  guile> foo
>  ERROR: Unbound variable: foo
>  ABORT: (unbound-variable)
> 
> No doubt this fails for the same reason this does:
> 
>  guile> (define 'foo "bar")
>  guile> foo
>  ERROR: Unbound variable: foo
>  ABORT: (unbound-variable)
> 
> What exactly happens when you 'define' a symbol?

I don't know what happens (in Guile), but I can tell
you what _should_ happen.  (In my humble opinion as
a demi-god of semantics.)

Scheme> (define 'foo "bar")

at a very early stage (like in the reader) this is

Scheme> (define (quote foo) "bar")

There are several possiblities

(1) error message in define
  (1a) Rude: Thou fool! |quote| is not a variable
  (1b) Polite: If you are sure you want to do that,
       write out the quote, don't use apostrophe
  (1c) Obscure: Bad variable in def_schkdt

(2) Redefine quote
  (2a) In a new local scope: 'a is a call to procedure
       |quote| with argument the value of |a|, argument is
       evaluated, ignored, and 'a -> "bar" if |a| is defined.
  (2b) Hell breaks loose: All uses of |quote| in previously
       defined system procedures now have a new meaning.
       e.g. depending on exact code for |abs|, maybe
       (abs -3) -> "bar"

But you are asking the wrong question.  Ask not
what happens when a symbol is defined, ask what
you can do to make the macro define an unquoted
variable.

   -- Keith

PS: Experiments with an old version of Guile indicate (2).
To distinguish (2a) from (2b) would require work or knowledge.

guile> (version)
"1.6.4"
guile>  (define 'foo "bar")
guile> (quote 3)
"bar"
guile> '3
"bar"
guile> 'foo
standard input:6:4096: While evaluating arguments to quote in expression (quote 
foo):
standard input:6:4096: Unbound variable: foo
ABORT: (unbound-variable)