Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

// why does this need interpolation to work?
@function capitalize( $string ) {
  @return '#{to-upper-case(str-slice($string, 1, 1))}' + str-slice($string, 2);
}
works {
  bar: capitalize( foo );
  bar: capitalize( 'bar' );
  bar: capitalize( "baz" );
}

// because this fails...
@function capitalize( $string ) {
  @return to-upper-case(str-slice($string, 1, 1)) + str-slice($string, 2);
}
fails {
  bar: capitalize( foo );
  bar: capitalize( 'bar' );
  bar: capitalize( "baz" );
}

SassMeister-output.css

works {
  bar: "Foo";
  bar: "Bar";
  bar: "Baz";
}

fails {
  bar: Foo;
  bar: "B"ar;
  bar: "B"az;
}