Re: How to create a higher order function?

[Leo Butler]

Marcin Borkowski <mbork@mbork.pl> writes:

> Hi all,
>
> assume that I want a function which "inverts" the meaning of a given
> predicate.  Something that would turn `<' into `>=' etc.
>
> So, here's a better (at least: working) version (B):
>
> ;;; -*- lexical-binding: nil; -*-
> (defun negate (fun)
>   "Return a function returning the logical opposite of FUN."
>   `(lambda (&rest args)
>     (not (apply ,(symbol-function fun) args))))

I think negate would need to be a defmacro in this case, not a defun.
This construction, as it is, would return a list, not a lambda function.

 `(foo ,X bar) is equivalent to (list 'foo X 'bar).

>
>
> It seems to work fine, even if it's a bit complicated.
>
> Of course, the more obvious way to do it is to use the first definition
> under lexical scope (C):
>
> ;;; -*- lexical-binding: t; -*-
> (defun negate (fun)
>   "Try to return a function returning the logical opposite of FUN."
>   (lambda (&rest args)
>     (not (apply fun args))))

Obviously C is better.

Q: I see you use mu4e. How do you get the code highlighted? (in my Gnus,
the background is distinctive blue).

Leo