How can I define a Yasnippet for #if AND if?

Question

I would like to define one snippet for a cc-mode if statement and a different one for the preprocessors #if statement. I tried just defining those two:

For if:

# -*- mode: snippet -*-
# name : if (...) { ... }
# key: if
# expand-env: ((yas-also-auto-indent-first-line t))
# --
if ( ${1:condition} ) {
    $0
}

For #if:

# -*- mode: snippet -*-
# name: #if
# key: #if
# expand-env: ((yas-wrap-around-region t))
# --
#if ${1:MODULE_NAME}
$0
#endif // $1

but when I try to complete #if, I get the completion for if. Directly calling M-x yas-insert-snippet with #if as parameter works perfectly...

Thanks, Markus

Answer 1

The main issue here is understanding the priorities yasnippet uses to determine what snippet it should expand.

If I understand the documentation correctly, this is controlled by the yas-key-syntaxes variable which by default is set to: ("w" "w_" "w_." "w_.()" yas-try-key-from-whitespace). This might look rather cryptic if you're not familiar with Emacs syntax classes, but it should be rather clear from the following description:

Yasnippet wants to skip an unknown number of characters backwards to find keywords to expand, so it tests each of the elements of yas-key-syntaxes in turn, using each combination of characters to determine what characters to skip:

• w Skip any piece of text that matches words.
• _ Skip any piece of text that matches symbols.
• . Skip any piece of text that matches punctuation.

In this case, you can fix your #if snippet by changing the yas-key-syntax priorities to try "w." before "w" or "w_" since # belong to the punctuation syntax class. I.e.,:

(setq yas-key-syntaxes '("w." "w" "w_" "w_." "w_.()" yas-try-key-from-whitespace))

Personally, I would bind the snippet to a different keyword, such as pif (pp for pre-processor) instead, but feel free to try it out.

It might also be useful to check out M-x describe-syntax, to figure out what characters belong to which syntax class in a particular major mode.