feat(uniq): initial commit of function

src/_uniq.scss

@@ -0,0 +1,37 @@
+@import 'reduce';
+
+///
+/// Returns a new list containing only one copy of each element in the original
+/// list. [`equals`](#equals) is used to determine equality.
+///
+/// @group list
+/// @param {Array} list The array to consider.
+/// @return {Array} The list of unique items.
+/// @example
+///
+///   uniq([1, 1, 2, 1]); //=> [1, 2]
+///   uniq([1, '1']);     //=> [1, '1']
+///   uniq([[42], [42]]); //=> [[42]]
+///
+
+@function uniq($list...) {
+    $result: ();
+
+    @if type-of($list) == arglist and length(nth($list, 1)) == 0 {
+        @return $result;
+    }
+
+    $_list: if(
+        type-of($list) == arglist and length($list) == 1,
+        reduce(append, (), nth($list, 1)),
+        $list
+    );
+    @each $item in $_list {
+        // should probably test with `get-function` for functions.
+        @if not index($result, $item) {
+            $result: append($result, $item);
+        }
+    }
+
+    @return $result;
+}

test/_uniq.spec.scss

@@ -0,0 +1,28 @@
+@import 'true';
+@import '../src/uniq';
+@import '../src/add';
+@import '../src/identity';
+
+@include describe('uniq [function]') {
+    @include it(
+        'returns a set from any array (i.e. purges duplicate elements)'
+    ) {
+        $list: [1, 2, 3, 1, 2, 3, 1, 2, 3];
+        @include assert-equal(uniq($list), (1 2 3));
+    }
+
+    @include it('keeps elements from the left') {
+        @include assert-equal(uniq([1, 2, 3, 4, 1]), (1 2 3 4));
+    }
+
+    @include it('returns an empty array for an empty array') {
+        @include assert-equal(uniq([]), ());
+    }
+
+    @include it('uses reference equality for functions') {
+        @include assert-equal(
+            length(uniq([add, identity, add, identity, add, identity])),
+            2
+        );
+    }
+}