Re: load in environment

[Jon Wilson]

Stephen Compall wrote:
> What other Guile state might you want to modify in the dynamic context 
> of a load, though?

Dynamic-wind looks like a quite good idea here.  I'll probably use 
that.  Thanks.

Well, I'm writing a function to load up some data from a file and stick 
it in a hash table.  The data file looks something like this:

-- begin data file --
(item "foo" "bar is a metasyntactic variable" 1)
(item "baz" "barn is a unit of cross section" 2)
(item "frob" "nitz is more or less meaningless" 86)
-- end data file --

Then I call a function to load this data.

(define (load-data filename)
 (let* ((my-table (make-hash-table))
        (item (lambda (name text number)
                      (hash-set! my-table name (make-item text number))))
        (m (make-module))
        (real-current-module (current-module)))
   (module-define! m 'item item)
   (set-current-module m)
   (load filename)
   (set-current-module real-current-module)
   my-table)))

[note: make-item is a record constructor.]

I certainly could just write code that reads in an expression from the 
file, and sticks the data from it into the hash table.  Probably safer 
than using load (nothing is evaluated), and perhaps better for other 
reasons as well.  But this way is very pretty (I think).  Additionally, 
if I add (module-use! m (null-environment 5)) or some such, then the 
user can generate the data programmatically, which sounds attractive 
(there isn't currently a good use-case for this, but I can picture some 
down the road perhaps).

I'm not sure if my-table is what you meant by "other guile state" that I 
might want to modify, but it is definitely something visible to the rest 
of the program which I want to modify by loading filename.  I'm a little 
bit mystified that I can modify my-table from inside (load filename) at 
all, since my-table is certainly not visible to the code inside 
filename.  I guess that I can because my-table is wrapped up in the 
closure of item.  I'm not really clear on the interaction between 
modules (especially set-current-module) and closures.  A closure inside 
of one module can apparently reference and modify things in quite 
another module altogether.  Maybe if it were not 1am, this would be more 
obvious.
Regards,
Jon