Trouble using a lambda in the key-translation-map

Trouble using a lambda in the key-translation-map

[Barry OReilly]

As the Elisp manual describes, one can bind a key to a function in the
key-translation-map, and use that function to programmatically
determine what to translate the key to.
  http://www.gnu.org/software/emacs/manual/html_node/elisp/Translation-Keymaps.html

I got this to work for functions defined with defun, but can't get it
to work with a lambda.  In an 'emacs -q' session, I put this in the
scratch buffer:
  (defun my-translate-key (prompt) (interactive) (kbd "C-c"))
  (define-key key-translation-map (kbd "C-e") (lambda (prompt)
(interactive) (kbd "C-c"))) ; Doesn't work
  (define-key key-translation-map (kbd "C-e") 'my-translate-key) ; Works

I evaluate the first and second sexp, and find C-e still goes to the
end of the line.  When I next execute the third sexp, C-e now
translates to C-c.  Why doesn't the second sexp enable the translation
from C-e to C-c?

Re: Trouble using a lambda in the key-translation-map

[Barry Margolin]

In article <mailman.5119.1342663755.855.help-gnu-emacs@gnu.org>,
 Barry OReilly <gundaetiapo@gmail.com> wrote:

> As the Elisp manual describes, one can bind a key to a function in the
> key-translation-map, and use that function to programmatically
> determine what to translate the key to.
>   http://www.gnu.org/software/emacs/manual/html_node/elisp/Translation-Keymaps
>   .html
> 
> I got this to work for functions defined with defun, but can't get it
> to work with a lambda.  In an 'emacs -q' session, I put this in the
> scratch buffer:
>   (defun my-translate-key (prompt) (interactive) (kbd "C-c"))
>   (define-key key-translation-map (kbd "C-e") (lambda (prompt)
> (interactive) (kbd "C-c"))) ; Doesn't work
>   (define-key key-translation-map (kbd "C-e") 'my-translate-key) ; Works
> 
> I evaluate the first and second sexp, and find C-e still goes to the
> end of the line.  When I next execute the third sexp, C-e now
> translates to C-c.  Why doesn't the second sexp enable the translation
> from C-e to C-c?

Look at the description of define-key -- the third argument has to be a 
SYMBOL for it to be treated as a function.  Lambda doesn't return a 
function, it returns a list.

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***

Re: Trouble using a lambda in the key-translation-map

[Barry OReilly]

>>   (defun my-translate-key (prompt) (interactive) (kbd "C-c"))
>>   (define-key key-translation-map (kbd "C-e") (lambda (prompt)
>> (interactive) (kbd "C-c"))) ; Doesn't work
>>   (define-key key-translation-map (kbd "C-e") 'my-translate-key) ; Works

> Look at the description of define-key -- the third argument has to be a
> SYMBOL for it to be treated as a function.  Lambda doesn't return a
> function, it returns a list.

Yet this works:
   (define-key global-map (kbd "C-e") (lambda () (interactive)
(message "Inside C-e's lambda")))
C-e displays the message without going to the end of the line, as expected.

I thought wherever functions are passed, a lambda could be passed
instead.  The Elisp manual corroborates this in section 12.7 Anonymous
Functions: "Anonymous functions are valid wherever function names
are.".  I don't see where my misunderstanding is.

I tried another thing in scratch:
  (progn
    (fset 'foo (lambda (prompt) (message "Inside lambda.") (kbd "C-c")))
    (define-key key-translation-map (kbd "C-e") 'foo)
    )

This behaves as I'd expect; I see "Inside lambda" and a translation to
C-c.  Thus it would seem the type returned from the lambda form is not
the problem.

The reason I ask is because I'm trying to create a generalized way to
make key translations that either translate to another key or to
itself based on an evaluated predicate.  My approach was:

(defun make-conditional-key-translation (key-from key-to translate-keys-p)
  "Make a Key Translation such that if the translate-keys-p function
returns true,
key-from translates to key-to, else key-from translates to itself.
translate-keys-p
takes no args. "
  (define-key key-translation-map key-from
              (lambda (prompt)
                      (if (funcall translate-keys-p) key-to key-from)))
  )
(defun my-translate-keys-p ()
  "Returns whether conditional key translations should be active.  See
make-conditional-key-translation function. "
  (or (evil-motion-state-p) (evil-normal-state-p) (evil-visual-state-p))
  )
(make-conditional-key-translation (kbd "ce") (kbd "C-e") 'my-translate-keys-p)