3 files changed, 125 insertions, 0 deletions
diff --git a/src/data/composite/data/index.js b/src/data/composite/data/index.js
index db1c37cc..e2927afd 100644
--- a/src/data/composite/data/index.js
+++ b/src/data/composite/data/index.js
@@ -11,3 +11,4 @@ export {default as withPropertiesFromObject} from './withPropertiesFromObject.js
 export {default as withPropertyFromList} from './withPropertyFromList.js';
 export {default as withPropertyFromObject} from './withPropertyFromObject.js';
 export {default as withUnflattenedList} from './withUnflattenedList.js';
+export {default as withUniqueItemsOnly} from './withUniqueItemsOnly.js';
diff --git a/src/data/composite/data/withUniqueItemsOnly.js b/src/data/composite/data/withUniqueItemsOnly.js
new file mode 100644
index 00000000..7ee08b08
--- /dev/null
+++ b/src/data/composite/data/withUniqueItemsOnly.js
@@ -0,0 +1,40 @@
+// Excludes duplicate items from a list and provides the results, overwriting
+// the list in-place, if possible.
+
+import {input, templateCompositeFrom} from '#composite';
+import {unique} from '#sugar';
+
+export default templateCompositeFrom({
+  annotation: `withUniqueItemsOnly`,
+
+  inputs: {
+    list: input({type: 'array'}),
+  },
+
+  outputs: ({
+    [input.staticDependency('list')]: list,
+  }) => [list ?? '#uniqueItems'],
+
+  steps: () => [
+    {
+      dependencies: [input('list')],
+      compute: (continuation, {
+        [input('list')]: list,
+      }) => continuation({
+        ['#values']:
+          unique(list),
+      }),
+    },
+
+    {
+      dependencies: ['#values', input.staticDependency('list')],
+      compute: (continuation, {
+        '#values': values,
+        [input.staticDependency('list')]: list,
+      }) => continuation({
+        [list ?? '#uniqueItems']:
+          values,
+      }),
+    },
+  ],
+});
diff --git a/test/unit/data/composite/data/withUniqueItemsOnly.js b/test/unit/data/composite/data/withUniqueItemsOnly.js
new file mode 100644
index 00000000..965b14b5
--- /dev/null
+++ b/test/unit/data/composite/data/withUniqueItemsOnly.js
@@ -0,0 +1,84 @@
+import t from 'tap';
+
+import {compositeFrom, input} from '#composite';
+import {exposeDependency} from '#composite/control-flow';
+import {withUniqueItemsOnly} from '#composite/data';
+
+t.test(`withUniqueItemsOnly: basic behavior`, t => {
+  t.plan(3);
+
+  const composite = compositeFrom({
+    compose: false,
+
+    steps: [
+      withUniqueItemsOnly({
+        list: 'list',
+      }),
+
+      exposeDependency({dependency: '#list'}),
+    ],
+  });
+
+  t.match(composite, {
+    expose: {
+      dependencies: ['list'],
+    },
+  });
+
+  t.same(composite.expose.compute({
+    list: ['apple', 'banana', 'banana', 'banana', 'apple', 'watermelon'],
+  }), ['apple', 'banana', 'watermelon']);
+
+  t.same(composite.expose.compute({
+    list: [],
+  }), []);
+});
+
+t.test(`withUniqueItemsOnly: output shapes & values`, t => {
+  t.plan(2 * 3 ** 1);
+
+  const dependencies = {
+    ['list_dependency']:
+      [1, 1, 2, 3, 3, 4, 'foo', false, false, 4],
+    [input('list_neither')]:
+      [8, 8, 7, 6, 6, 5, 'bar', true, true, 5],
+  };
+
+  const mapLevel1 = [
+    ['list_dependency', {
+      '#list_dependency': [1, 2, 3, 4, 'foo', false],
+    }],
+    [input.value([-1, -1, 'interesting', 'very', 'interesting']), {
+      '#uniqueItems': [-1, 'interesting', 'very'],
+    }],
+    [input('list_neither'), {
+      '#uniqueItems': [8, 7, 6, 5, 'bar', true],
+    }],
+  ];
+
+  for (const [listInput, outputDict] of mapLevel1) {
+    const step = withUniqueItemsOnly({
+      list: listInput,
+    });
+
+    quickCheckOutputs(step, outputDict);
+  }
+
+  function quickCheckOutputs(step, outputDict) {
+    t.same(
+      Object.keys(step.toDescription().outputs),
+      Object.keys(outputDict));
+
+    const composite = compositeFrom({
+      compose: false,
+      steps: [step, {
+        dependencies: Object.keys(outputDict),
+        compute: dependencies => dependencies,
+      }],
+    });
+
+    t.same(
+      composite.expose.compute(dependencies),
+      outputDict);
+  }
+});