Yasnippet conditional template depending on field value

Question

I'm having an issue where I would like to be able to create a template which will conditionally insert a string into the template based on the value of one of the fields.

This means I want some sort of embedded elisp (or elisp function call) to evaluate if one of my fields has the value 0. If it does not then I want to include a function definition in the snippet. If the field is 0 then there is no reason to include this function definition.

This is what I have so far but it seems to matter what the string is included and the concatenation fails.

$>registerFn(id, &$1FieldEnd, $1FieldOtherEnd,
$>                ${5:$1Handler}, 0);
`(if (string= (yas-field-value 5) "0")
        (
        " ")
        (
concat (concat "\nstatic void " (concat (yas-field-value 5) "(void *argPtr)"))
"\n
{
/* handlerImplementation */
}
")
)`

Answer 1

On a more general note, here's an example that sets b to the value of a if a is not 0.

# -*- mode: snippet -*-
# name: test
# key: test
# --

a = ${1:0}
b = ${1:$(unless (equal yas-text "0") yas-text)}
$0

Answer 2

I managed to solve this by simply putting the following in my doom.d/config.el

(with-eval-after-load 'yasnippet
        (defun maybe_notify_snip (snip_str)
            (if (string= snip_str "0")
                " "
                (concat (concat "\nstatic void " (concat snip_str "(void *argPtr)"))
                 "\n
                 {
                 /* handler implementation */
                 }
                 "))))

and then using it inside the template as such:

${5:$(maybe_notify_snip yas-text)}

This seems to ensure that the snippet is updated such that if I enter 0 for field 5 then we don't include a function implementation snippet but as soon as I enter something in filed 5 the implementation appears