fix: fix libsass 4 call deprecation warning #63

[ncoden]

Fix the following deprecation warning:

DEPRECATION WARNING: Passing a string to call() is deprecated and will be illegal
in Sass 4.0. Use call(get-function("fade")) instead.

Call call() with a function retrieved by the new function get-function.

Changes:

• add sl-safe-call-function polyfill for call() with support of both function (LibSass >= 3.5) and function name string (LibSass < 3.5)
• add a warning message about the deprecated usage of function name string in LibSass >= 3.5
• use sl-safe-call-function instead of call()

Closes #63

[ncoden]

@hugogiraudel

[KittyGiraudel]

Shall this be prefixed with sl-?

[ncoden]

This should, yes.

[xzyfer]

There's an alternative approach that would maintain BC with older Sass. You could instead have a safe-call-function. See a very simplified example below.

@function safe-call-function($name, ...$args) {
  @if function-exists('get-function') && type-of($name) == string {
    @return call(get-function($name),  ...$args);
  }
  @return call($name, ...$args);
}

This approach will work on all version Sass so theres no need for the @error.

[KittyGiraudel]

That’s a good idea (expect for && that should be and in Sass ;)), let’s do this. :)

[ncoden]

@hugogiraudel @xzyfer I applied the required changes. See PR description

[ncoden]

hmmm actually I found a more suitable way to make these checks. See foundation/motion-ui#117. I think I'll make a package from it.

[ncoden]

Done. See https://github.com/ncoden/sassy-functions. @hugogiraudel @xzyfer I'll use it for this pull request, after I added some tests.