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?

[Barry Margolin]

In article <mailman.5119.13426637...@gnu.org>,

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, bar...@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***

[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)