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Re: Programmatic let

From: Lluís
Subject: Re: Programmatic let
Date: Wed, 05 Dec 2012 21:48:03 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3.50 (gnu/linux)

Grégoire Jadi writes:

> Lluís <address@hidden> writes:
>> My elisp foo is quite poor, so this might well be a completely stupid 
>> approach
>> to the problem.
>> I have a callback that gets automatically called, and uses some 
>> orgmode-specific
>> routines whose behaviour can be tuned through some customizable variables.
>> The usual approach in orgmode is:
>> (let ((org-v1 ...) (org-v2 ...)) (org-func ...))
>> As my function is automatically called (as a response to the D-Bus message), 
>> I'd
>> like to let the user specify a let-like form to let her temporarily override 
>> the
>> behaviour of the `org-func' that my package invokes:
>> (setq my-cb-org-vars '((org-v1 ...) (org-v2 ...)))
>> (defun my-cb (...)
>> (programmatic-let my-cb-org-vars
>> (org-func ...)))
>> Is it possible to implement `programmatic-let'? And if so, can you throw me a
>> pointer to the appropriate docs?

> You could use progv:
>> (progv SYMBOLS VALUES &rest BODY)
>> Bind SYMBOLS to VALUES dynamically in BODY.
>> The forms SYMBOLS and VALUES are evaluated, and must evaluate to lists.
>> Each symbol in the first list is bound to the corresponding value in the
>> second list (or to nil if VALUES is shorter than SYMBOLS); then the
>> BODY forms are executed and their result is returned.  This is much like
>> a `let' form, except that the list of symbols can be computed at run-time.

> However, given a defined set of variables, you could parse
> my-cb-org-vars yourself:

> (defvar my-cb-org-vars '((x ...) (y ...)))
> (defun my-cb (...)
>   (let ((x (cdr (assoc 'x my-cb-org-vars)))
>         (y (cdr (assoc 'y my-cb-org-vars))))
>     ....))

Thanks a lot, `progv' is exactly what I want, given that the list of vars is not
known a priori.


 "And it's much the same thing with knowledge, for whenever you learn
 something new, the whole world becomes that much richer."
 -- The Princess of Pure Reason, as told by Norton Juster in The Phantom

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