How to defalias lambda?

Question

I have this in my .emacs:

(defalias 'λ 'lambda)

which works fine for simple things like (funcall (λ (x) x) 1).

But when I do something like (org-add-link-type "foo" (λ (s) (message s))) or (add-to-list 'auto-mode-alist '("foo" . (λ () (message "bar")))), it doesn't work and I get

org-open-at-point: Invalid function: (λ (s) (message s))

and

File mode specification error: (invalid-function (λ nil (message "bar")))

respectively.

Does anybody know what's wrong here?

Answer 1

With some help from lunaryorn on reddit, I guess I have been able to understand why you are observing the "wierd" behaviour.

The problem is that you are quoting the expression

'("foo" . (λ () (message "bar")))

Which is equivalent to the form

(cons "foo" '(λ () (message "bar")))

Now when emacs opens a file with extension "foo", it does something like the following

(funcall '(λ () (message "bar")))

Notice the extra quote, before (λ, this obviously is not valid function and you get the error Invalid function: ... But then why does ("foo" . (lambda () (message "bar"))) work, this is explained by lunaryorn's observation

A “lambda list”, i.e. a list whose car is lambda, is a valid function, too

So '(lambda () (message "bar")) is a valid function, this can be verified by the following code

(functionp (lambda () "hello"))  => t
(functionp (λ () "hello"))       => t
(functionp '(lambda () "hello")) => t
(functionp '(λ () "hello"))      => nil

So the solution would be to simply not quote the expression use the following instead

(add-to-list 'auto-mode-alist (cons "foo" (λ () (bar))))

Answer 2

It looks like the problem is not with the defalias, but rather where and how you're calling λ. funcall takes, as its arguments, a function and that function's arguments, so your funcall example works just fine.

Both org-add-link-type and the auto-mode-alist, however, expect symbols that contain the relevant functions. Hence, building off of your examples, the following should work:

(defun a-silly-fnx (s)
  (message s))
(defalias #'my-link-alias #'a-silly-fnx)
(org-add-link-type "foo" #'my-link-alias)

(defun a-tester-fnx ()
  (message "Testing!")
  (sit-for 2))
(defalias #'my-alist-alias #'a-tester-fnx)
(add-to-list 'auto-mode-alist '("foo" . my-alist-alias))

If you're mostly looking to have λ show up in your buffer, consider trying out prettify-symbols-mode, which will display lambda as λ without changing th buffer's text.