9

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?

rtrn
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    Not an answer to the question about the `defalias`, but you might want to check out [`prettify-symbols-mode`](http://ergoemacs.org/emacs/emacs_pretty_lambda.html), which, among other things, will allow you to display `lambda` as `λ` without actually changing the underlying text. – Dan Dec 15 '14 at 12:13
  • A simple library that prettifies *only* `lambda` (to `λ`): [`pretty-lambdada.el`](http://www.emacswiki.org/emacs-en/download/pretty-lambdada.el). – Drew Dec 15 '14 at 15:48
  • If you just want pretty symbols then use https://github.com/akatov/pretty-mode – grettke Dec 16 '14 at 04:25

2 Answers2

12

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))))
Community
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Iqbal Ansari
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3

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.

Dan
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  • The weird thing is that everything works fine when I use `lambda` directly. – rtrn Dec 15 '14 at 12:57
  • @rtrn: ah, good point. I'm guessing it's because the `lambda` macro returns a self-quoted `lambda` special form which the alias isn't picking up, but there might be more black magic here. Summon @Malabarba. – Dan Dec 15 '14 at 13:09