fboundp and functionp: what's the difference?
reddit / emacs

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NightStruck · 2022-08-04 05:43

i'm kinda confused on what makes `fboundp` and `functionp`
functionally different from each other. it seems to me that i can just
swap between the two whenever.

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ieure · 2022-08-04 06:57

`FUNCTIONP` tells you if *its argument is* a function value; `FBOUNDP`
tells you if *a symbol has* a function value bound to it.

    ELISP> (functionp 'car)
    t
    ELISP> (fboundp 'car)
    t
    ELISP> (functionp (lambda ()))
    t
    ELISP> (fboundp (lambda ()))
    *** Eval error ***  Wrong type argument: symbolp, (closure (t)
nil)

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organic-bookworm · 2022-08-04 08:07

I'll add a bit.

And `(fboundp 'sym)` is ~~functionally equivalent~~ to `(functionp
(symbol-function 'sym))`, so you could say `functionp` is a ~~more
generic function~~.

EDIT: See [u/xtifr](https://www.reddit.com/r/emacs/comments/wft3je/com
ment/iiwci66) comment.

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xtifr · 2022-08-04 10:21

Also, fboundp tells you whether the symbol's function cell contains
*something*, not whether that something is a valid function.

    ELISP> (fset 'foo "abc")
    "abc"
    ELISP> (fboundp 'foo)
    t
    ELISP> (functionp 'foo)
    nil

(The docs assure me that there are reasons for putting a keymap or
keyboard macro in a symbol's function cell, but I'm fuzzy on the
details.)

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NightStruck · 2022-08-04 10:53

thanks everyone, appreciate the examples!

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organic-bookworm · 2022-08-04 11:09

Good point.

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mickeyp · 2022-08-04 12:01

> (The docs assure me that there are reasons for putting a keymap or
keyboard macro in a symbol's function cell, but I'm fuzzy on the
details.)

You can`fset` keyboard macros with `kmacro-lambda-form` and a vector
of the actions-to-be. That's what `insert-kbd-macro` code generates
for you.