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SassMeister-input.scss

// ----
// libsass (v3.2.4)
// Sass (v3.4.13)
// Compass (v1.0.3)
// ----

///
/// Customization modules
///
/// @type {Map}
///
$customizer: (
	"theme-1": (
		"primary": (
			"lightest": t1pc1,
			"light": t1pc2,
			"base": t1pc3,
			"dark": t1pc4,
			"darkest": t1pc5,
		),
		"complementary": (
			"light": t1cc1,
			"base": t1cc2,
			"dark": t1cc3,
		),
		"gray": (
			"light": t1gc1,
			"base": t1gc2,
			"dark": t1gc3,
		),
		"black": t1b,
		"white": t1w,
	),
  "theme-2": (
		"primary": (
			"lightest": t2pc1,
			"light": t2pc2,
			"base": t2pc3,
			"dark": t2pc4,
			"darkest": t2pc5,
		),
		"complementary": (
			"light": t2cc1,
			"base": t2cc2,
			"dark": t2cc3,
		),
		"gray": (
			"light": t2gc1,
			"base": t2gc2,
			"dark": t2gc3,
		),
		"black": t2b,
		"white": t2w,
	),
) !global;

///
/// Helper functions
///

///
/// Fetch value from map
///
/// @param {Map}     $map  - map to fetch value from
/// @param {ArgList} $keys - list of keys to traverse
///
/// @return {*}
///
@function map-fetch($map, $keys...) {
	$result: $map;

	@each $key in $keys {
		@if $result {
			$result: type-of($result) == "map" and map-has-key($result, $key) and map-get($result, $key) or false;
		} @else {
			@return false;
		}
	}

	@return $result;
}

///
/// Check if key exists in map
///
/// @param {Map}    $map   - map to search
/// @param {String} $value - key to search for
///
/// @return {Bool}
///
@function exists($map, $value){
	$is-map: type-of($map) == "map";
	$top-has-key: $is-map and map-has-key($map, $value) or false;

	@if $top-has-key {
		@return true;
	} @else if $is-map {
		@each $m, $k in $map {
			@if exists($k, $value) {
				@return true;
			}
		}
	}

	@return false;
}

///
/// Map of instances
///
/// @type {Map}
///
$customizer-instances: () !global;

///
/// Create new customizable instance
///
/// @param {Map}    $arg    - map of customizable property->value pairs
/// @param {String} $module - module to pull property values from
///
/// @return {Map} $customizer-instances
///
@function new-customizer-instance($args, $module) {
	// Empty argument map
	$instance-properties: ();

	// Run through each argument individually
	@each $arg in $args {
		$property: nth($arg, 1);
		$value: nth($arg, 2);

		// Build temp map to merge into $instance-properties for each property
		$temp: (
			"#{$property}": (
				"module": $module,
				"value": $value
			)
		);

		// Merge into argument map
		$instance-properties: map-merge($instance-properties, $temp);
	}

	// Create new instance map for selector, save properties
	$customizer-instance: (
		"#{&}": (
			$instance-properties
		)
	);
	
	// Check if selector already exists, if so, merge instance vars
	@if exists($customizer-instances, "#{&}") {
	  $self: map-fetch($customizer-instances, "#{&}");
	  
	  // Merge new vars into self, then merge into main map
	  @return map-merge($customizer-instances, map-merge($self, $customizer-instance));
	}

	// Merge into main map
	@return map-merge($customizer-instances, $customizer-instance);
}

///
/// Return value for property based on passed module
///
/// @param {List}   $args   - list of keys for customizable property
/// @param {String} $module - module to pull property values from
///
/// @return {*} - $value from $module
///
@function use-module($args, $module) {
	// Module to use
	$use: $module;
	// Make sure all submodules exist
	$exists: true;
	// Grab length of arguments
	$length: length($args);

	// Break down arguments into list to pass into map-fetch
	@each $arg in $args {
		$use: append($use, $arg);
	}

	// Check if sub-modules exist
	@if $length > 1 {
		@for $i from 1 through $length {
			@if not exists($customizer, nth($args, $i)) {
				$exists: false;
			}
		}
	}

	@if $exists {
		// Grab value from module by passing in newly built list
		@return map-fetch($customizer, $use...);
	} @else {
		// One or more of the modules were not found, throw error
		@warn "Invalid arguments: #{$use}. One or more module or sub-module not found.";
		@return $exists;
	}
}

///
/// Create new customizable instance
///
/// @param {Map}    $args   - map of customizable property->value pairs
/// @param {String} $module - module to pull property values from
///
/// @return {Map} $customizer-instances
///
@mixin new-customizer-instance($args, $module) {
	$customizer-instances: new-customizer-instance($args, $module) !global;
}

/// Create new customizable properties, save to instance map
///
/// @param {Map}    $args   - map of customizable property->value pairs
/// @param {String} $module - module to pull property values from
///
/// @output $property->$value pairs for each argument
///
@mixin customizer($args, $uses: null) {
  
	// Make sure argument is a map
	@if type-of($args) == "map" {

		// Use module? Expecting module to exist
		@if $uses != null {

			// Check if module exists
			@if exists($customizer, $uses) {

				// Save arguments to instance map
				@include new-customizer-instance($args, $uses);

				// Run through each argument individually
				@each $arg in $args {
					// Break up argument into property->value
					$property: nth($arg, 1);
					$value: nth($arg, 2);

					// Check if sub-module exists
					@if type-of($value) == "list" or exists($customizer, $value) {
						// Get values from sub-module
						$value: use-module($value, $uses);
					// Sub-module did not exist, throw error
					} @else {
						@warn "Invalid argument: #{$value}. Sub-module was not found.";
					}

					// Output styles
					#{$property}: $value;
				}

			// Module did not exist, throw error
			} @else {
				@warn "Invalid argument: #{$uses}. Module was not found.";
			}

		// No module specified, expecting plain CSS
		} @else {

			// Loop through each argument individually
			@each $arg in $args {

				// Break up argument into property->value
				$property: nth($arg, 1);
				$value: nth($arg, 2);

				// Output styles
				#{$property}: $value;
			}

			// Warn that customization mixin shouldn't be used without a module
			@warn "The customization mixin should not be used without specifying a module to use.";
		}

	// Argument was not a map, throw error
	} @else {
		@warn "Invalid argument: #{$args}. Argument type is not a map.";
	}
}

///
/// Themify
///
@mixin themify ($customizer: $customizer) {
  @each $theme, $map in $customizer {
    .#{$theme} &  {
      
      // Save the current theme to a global variable so
      //  that we can use it outside of themify()
      $theme: $theme !global;
      
      @content;
      
      // After we're done with it, reset the current active
      //  theme to null
      $theme: null !global;
    }
  }
}


///
/// Tests
///
/// Normal usage of customizer
// .customizer {
//   @include customizer(
//     $args: (
//       color: "white",
//       background: "primary" "darkest",
//       border-color: "complementary" "base"
//     ), 
//     $uses: "theme-1"
//   );
// }

// Attempt to use themes through "themify"
//  but fails because of a missing variable that needs to be passed to $uses
.side-nav {
  @include themify() {
    @include customizer(
      $args: (
        color: "primary" "dark"
      ),
      $uses: $theme // This is a global variable
    );
  }
}

output {
  instances: inspect($customizer-instances);
}

SassMeister-output.css

.theme-1 .side-nav {
  color: t1pc4;
}
.theme-2 .side-nav {
  color: t2pc4;
}

output {
  instances: (".theme-1 .side-nav": ("color": ("module": "theme-1", "value": "primary" "dark")), ".theme-2 .side-nav": ("color": ("module": "theme-2", "value": "primary" "dark")));
}