A variable that holds a string which may be the name of a variable.

A variable that holds a string which may be the name of a variable.

[Richard Shann]

I am stuck on one of those symbol/variable-name-in-a-string things
again:

(define mything "display")
(display (eval-string mything))

that's fine. But can I test that the string in mything is the name of a
variable before doing the eval-string and finding out the hard way? I've
been doing (symbol? mything) etc, and going witless. Do I have to do all
that catch stuff?

The situation is that I can guess (programmatically construct that is)
the name of the variable, but it may not be defined. If it isn't I will
just say so, but if it is I want the string it is defined to hold.

Any help much appreciated!

Richard Shann

Re: A variable that holds a string which may be the name of a variable.

[Neil Jerram]

Richard Shann <richard.shann@virgin.net> writes:

> I am stuck on one of those symbol/variable-name-in-a-string things
> again:
>
> (define mything "display")
> (display (eval-string mything))
>
> that's fine. But can I test that the string in mything is the name of a
> variable before doing the eval-string and finding out the hard way? I've
> been doing (symbol? mything) etc, and going witless. Do I have to do all
> that catch stuff?
>
> The situation is that I can guess (programmatically construct that is)
> the name of the variable, but it may not be defined. If it isn't I will
> just say so, but if it is I want the string it is defined to hold.
>
> Any help much appreciated!

Hi Richard... :-)

You can use defined? to find out if an arbitrary symbol is defined (in
the current module).

It takes a symbol, so for example:

  (let ((sym (with-input-from-string mything read)))
    (if (defined? sym)
        (eval sym (current-module))
        (display "Sorry, that isn't defined")))

Does that help?

Regards,
        Neil

Re: A variable that holds a string which may be the name of a variable.

[Linas Vepstas]

2009/12/2 Neil Jerram <neil@ossau.uklinux.net>:
Neil,

For my own edification -- is tehre any difference between

>  (let ((sym (with-input-from-string mything read)))

and

(let ((sym (string->symbol mything)))

?

--linas

Re: A variable that holds a string which may be the name of a variable.

[Neil Jerram]

Linas Vepstas <linasvepstas@gmail.com> writes:

> 2009/12/2 Neil Jerram <neil@ossau.uklinux.net>:
> Neil,
>
> For my own edification -- is tehre any difference between
>
>>  (let ((sym (with-input-from-string mything read)))
>
> and
>
> (let ((sym (string->symbol mything)))

In the case where mything is a string consisting of symbol syntax
characters, no.

Maybe you're then suggesting that Richard could use string->symbol
instead - in which case I agree.  Using with-input-from-string and read
is a bit of overkill; I forgot about string->symbol.

Regards,
        Neil

Re: A variable that holds a string which may be the name of a variable.

[Richard Shann]

On Thu, 2009-12-03 at 18:56 +0000, Neil Jerram wrote:
> Linas Vepstas <linasvepstas@gmail.com> writes:
> 
> > 2009/12/2 Neil Jerram <neil@ossau.uklinux.net>:
> > Neil,
> >
> > For my own edification -- is tehre any difference between
> >
> >>  (let ((sym (with-input-from-string mything read)))
> >
> > and
> >
> > (let ((sym (string->symbol mything)))
> 
> In the case where mything is a string consisting of symbol syntax
> characters, no.

In fact, I was somewhat interested that it should be fairly bomb proof
to the contents of mything. So I tried Neil's code on various dangerous
looking things...
I hadn't spotted string->symbol, else I might have got there unaided; I
had a hazy idea that that was what I was trying to do. Gradually the
mists are clearing - thank you.

Richard

Re: A variable that holds a string which may be the name of a variable.

[Linas Vepstas]

2009/12/2 Richard Shann <richard.shann@virgin.net>:
> I am stuck on one of those symbol/variable-name-in-a-string things
> again:
>
> (define mything "display")
> (display (eval-string mything))
>
> that's fine. But can I test that the string in mything is the name of a
> variable before doing the eval-string and finding out the hard way? I've
> been doing (symbol? mything) etc, and going witless. Do I have to do all
> that catch stuff?

If I understand you correctly, no. That's because the
thing going into eval-string can be arbitrarily complex --
e.g. (eval-string "(define foo (lambda (x) (+ x y))) (define (bar
z y) (foo (z)) (bar 2 2)")  and there's no particular way to know
if evaluating that won't throw some error. -- really, you have to
evaluate it to find out.

If you want to find out if some particular variable is bound
to a proceedure, you can try (proceedure? thing) see e.g.
http://www.gnu.org/software/guile/manual/html_node/Procedure-Properties.html

If you want to know if a variable is bound, try defined?  e.g.

guile> (defined? 'x)
#f
guile> (defined? (string->symbol "x"))
#f
guile> (define x 42)
guile> (defined? 'x)
#t
guile> (defined? (string->symbol "x"))
#t

--linas


--linas

Re: A variable that holds a string which may be the name of a variable.

[Richard Shann]

Thanks to all guileans who have responded. I seem to have a workable
solution based on Neil's code. The thing I hadn't grokked was the
existence of the with-input-from-string procedure. I had been messing
with defined? and symbol? but got nowhere.

FWIW the core bit of code I ended up with takes the name of a command,
<name> and looks for a string in a variable called Help-d-<name> if it
exists:

(set! help (string-append "Help-d-" name))	  
	  (let ((sym (with-input-from-string help read)))
	    (if (defined? sym)
		(set! help (eval sym (current-module)))
		(set! help "No help")
		))

This seems to cover all the cases. I haven't really got to the bottom of
what that with-input-from-string procedure does - I got as far as the
documentation for read which mentions an s-expression. Elsewhere I have
seen references to "forms", perhaps there is something establishing some
basic terminology that I have missed...

But thanks a lot - I am on my way again...

Richard

Re: A variable that holds a string which may be the name of a variable.

[Neil Jerram]

Richard Shann <richard.shann@virgin.net> writes:

> Thanks to all guileans who have responded. I seem to have a workable
> solution based on Neil's code. The thing I hadn't grokked was the
> existence of the with-input-from-string procedure. I had been messing
> with defined? and symbol? but got nowhere.

I would say the magic is more in `read' than in
`with-input-from-string'.

`read' is responsible for the conversion from
something-that-looks-like-code to actual code - which in your case is a
single symbol.  `with-input-from-string' just tells `read' that the
something-that-looks-like-code is a string.

Regards,
        Neil