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Re: load in environment

From: Jon Wilson
Subject: Re: load in environment
Date: Fri, 06 Jul 2007 01:19:06 -0400
User-agent: Thunderbird (X11/20070604)

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)

[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.

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