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Re: Concern around use of eval
From: |
Tassilo Horn |
Subject: |
Re: Concern around use of eval |
Date: |
Thu, 19 Mar 2015 09:00:06 +0100 |
User-agent: |
Gnus/5.130012 (Ma Gnus v0.12) Emacs/25.0.50 (gnu/linux) |
Les Harris <les@lesharris.com> writes:
Hi Les,
> ,----
> | (defvar lh/labels '("label1" "label2" "label3"))
> |
> | (defmacro lh/gen-predicate (label)
> | `(defun ,(intern (concat "lh/" label "-p")) ()
> | (member ,label *lh/system-label-store*)))
> |
> | (defun lh/define-predicates ()
> | (dolist (label lh/labels)
> | (eval `(lh/gen-predicate ,label))))
> | (lh/define-predicates)
> `----
>
> Now this all works fine and I get my auto-generated predicates, so
> success. My question/concern/niggle is around the use of (eval) in
> lh/define-predicates. If I don't put eval in there then the defun the
> macro evaluates into never gets evaluated itself. Stylistically, is
> there a better way to do this or am I just being weird about (eval) and
> should just get over it?
Well, in case the `lh/label' value is available at compile-time then
`lh/define-predicates' can also be a macro and you can go like this:
--8<---------------cut here---------------start------------->8---
(defvar lh/labels '("label1" "label2" "label3"))
(defmacro lh/gen-predicate (label)
`(defun ,(intern (concat "lh/" label "-p")) ()
(member ,label *lh/system-label-store*)))
(defmacro lh/define-predicates ()
`(progn
,@(mapcar (lambda (label)
`(lh/gen-predicate ,label))
lh/labels)))
(lh/define-predicates)
--8<---------------cut here---------------end--------------->8---
But if you know your labels only at runtime, then I think there's no way
around using `eval'.
Bye,
Tassilo