data: withLengthOfList

2 files changed, 56 insertions, 0 deletions

diff --git a/src/data/composite/data/index.js b/src/data/composite/data/index.js
index 46a3dc81..05b59445 100644
--- a/src/data/composite/data/index.js
+++ b/src/data/composite/data/index.js
@@ -20,6 +20,7 @@ export {default as withMappedList} from './withMappedList.js';
 export {default as withSortedList} from './withSortedList.js';
 export {default as withStretchedList} from './withStretchedList.js';
 
+export {default as withLengthOfList} from './withLengthOfList.js';
 export {default as withPropertyFromList} from './withPropertyFromList.js';
 export {default as withPropertiesFromList} from './withPropertiesFromList.js';
 
diff --git a/src/data/composite/data/withLengthOfList.js b/src/data/composite/data/withLengthOfList.js
new file mode 100644
index 00000000..efc3ecae
--- /dev/null
+++ b/src/data/composite/data/withLengthOfList.js
@@ -0,0 +1,55 @@
+import {input, templateCompositeFrom} from '#composite';
+import {stitchArrays} from '#sugar';
+
+function getOutputName({
+  [input.staticDependency('list')]: list,
+}) {
+  if (list && list.startsWith('#')) {
+    return `${list}.length`;
+  } else if (list) {
+    return `#${list}.length`;
+  } else {
+    return '#length';
+  }
+}
+
+export default templateCompositeFrom({
+  annotation: `withMappedList`,
+
+  inputs: {
+    list: input({type: 'array'}),
+  },
+
+  outputs: inputs => [getOutputName(inputs)],
+
+  steps: () => [
+    {
+      dependencies: [input.staticDependency('list')],
+      compute: (continuation, inputs) =>
+        continuation({'#output': getOutputName(inputs)}),
+    },
+
+    {
+      dependencies: [input('list')],
+      compute: (continuation, {
+        [input('list')]: list,
+      }) => continuation({
+        ['#value']:
+          (list === null
+            ? null
+            : list.length),
+      }),
+    },
+
+    {
+      dependencies: ['#output', '#value'],
+
+      compute: (continuation, {
+        ['#output']: output,
+        ['#value']: value,
+      }) => continuation({
+        [output]: value,
+      }),
+    },
+  ],
+});