shadow-callbag

A utility for implementing shadow callbags inside tallbags.

npm install shadow-callbag

API

Factory: makeShadow(name, sourceShadow)

• name is a string that identifies the current tallbag operator or source
• sourceShadow is the shadow callbag that comes from the source tallbag, when implementing an operator for tallbags

This function returns a shadow callbag. Whenever the tallbag delivers data, you should also deliver that same data to the shadow callbag. See example below.

Example

Implementing the tallbag-map (diffing with callbag-map):

+import makeShadow from 'shadow-callbag';
+// or
+// const makeShadow = require('shadow-callbag').default;

 const map = f => source => (start, sink) => {
   if (start !== 0) return;
-  source(0, (t, d) => {
+  let shadow;
+  source(0, (t, d, s) => {
     if (t === 0) {
+      shadow = makeShadow('map', s);
-      sink(0, d);
+      sink(0, d, shadow);
     } else if (t === 1) {
       const y = f(d);
+      shadow && shadow(1, y);
       sink(1, y);
     } else {
       sink(t, d);
     }
   });
 };

Metadata

The metadata produced by these shadow callbags follows the JSON type below:

type Metadata = {
  name: string;
  source?: Metadata | Array<Metadata> | undefined;
  data?: string | number | object;
  timestamp?: number;
};

Name is a label or identifier for the current tallbag.

Source is the metadata for the parent tallbag in a chain (directed acyclic graph) of tallbags, when such parent exists.

Data and timestamp describe the most recent delivery of data by this current tallbag.

Example metadata object displaying the chain of operators, interval-->map-->delay-->forEach, where only the root tallbag has had a previous delivery of data:

{
  "source": {
    "source": {
      "source": {
        "name": "interval",
        "data": 0,
        "timestamp": 1548507136457
      },
      "name": "map"
    },
    "name": "delay"
  },
  "name": "forEach"
}

Example metadata object for the same chain of operators, where all tallbags in the chain have delivered something (pay attention to the monotonic ordering of timestamps):

{
  "source": {
    "source": {
      "source": {
        "name": "interval",
        "data": 0,
        "timestamp": 1548507136457
      },
      "name": "map",
      "data": 0,
      "timestamp": 1548507136458
    },
    "name": "delay",
    "data": 0,
    "timestamp": 1548507136961
  },
  "name": "forEach",
  "data": 0,
  "timestamp": 1548507136961
}