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SassMeister-input.scss

// ----
// Sass (v3.3.9)
// Compass (v1.0.0.alpha.20)
// ----

// Dependency checker
//
// @param {ArgList} $functions - list of functions to check for
//
// @return {Bool} whether or not there are missing dependencies

@function missing-dependencies ($functions...) {
  $missing-dependencies: ();
  
  @each $function in $functions {
    @if not function-exists($function) {
      $missing-dependencies: append($missing-dependencies, $function, comma);
    }
  }
  
  @if length($missing-dependencies) > 0 {
    @warn "Unmet dependencies! The following functions are required: #{$missing-dependencies}.";
  }
  
  @return $missing-dependencies != 0;
}

// @alias missing-dependencies

@function dep($functions...) {
  @return missing-dependencies($functions...);
}

@function existing-function() {
  @return true;
}

// @requires inexisting-function
// @requires existing-function

@function dummy() {
  @if missing-dependencies("existing-function", "inexisting-function") == true {
    @return null;
  }

  // Function core
  @return "dummy result";
}

test {
  sass: dummy();
}