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Re: Programmatic let

From: Grégoire Jadi
Subject: Re: Programmatic let
Date: Wed, 05 Dec 2012 21:37:59 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3.50 (gnu/linux)

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))))

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