from: Accept a callback function.

doc/API.md

@@ -12,7 +12,7 @@
     - [Channel([bufferLength = 0]) -> Channel](#channelbufferlength--0---channel)
     - [Channel.isChannel(value) -> Boolean](#channelischannelvalue---boolean)
     - [Channel.of(...values) -> Channel](#channelofvalues---channel)
-    - [Channel.from(iterable | stream.Readable) -> Channel](#channelfromiterable--streamreadable---channel)
+    - [Channel.from(callback | iterable | stream.Readable) -> Channel](#channelfromcallback--iterable--streamreadable---channel)
   - [Channel Object](#channel-object)
     - [every(callbackfn[, thisArg]) -> async Boolean](#everycallbackfn-thisarg---async-boolean)
     - [filter(callbackfn[, thisArg]) -> Channel](#filtercallbackfn-thisarg---channel)
@@ -164,10 +164,14 @@ Return `true` if `value` is a channel, `false` otherwise.
 
 Push `values` into a new channel and then close it.
 
-### Channel.from(iterable | stream.Readable) -> Channel
+### Channel.from(callback | iterable | stream.Readable) -> Channel
 
-Create a new `Channel` from an iterable or a [Node.js readable
-stream](https://nodejs.org/api/stream.html#stream_readable_streams).
+Create a new `Channel` from a callback function, an iterable, or a [Node.js
+readable stream](https://nodejs.org/api/stream.html#stream_readable_streams).
+
+If given a callback function, call the function repeatedly to obtain values for
+pushing into the channel.  Close the channel when the function returns
+`undefined`.
 
 ## Channel Object
 

lib/index.js

@@ -308,30 +308,44 @@ const Channel = function(length = 0) {
 
 Channel.from = values => {
   const channel = Channel();
+
   (async () => {
+    // iterable
     try {
-      // iterable
       for (let value of values) {
         await channel.push(value);
       }
 
       await channel.close();
     } catch (exception) {
-      // Assume it's a Node.js stream.readable.
+      // callback function
+      try {
+        for (;;) {
+          const value = values();
 
-      values.on("readable", async () => {
-        while (true) {
-          const data = values.read();
-
-          if (data === null) {
+          if (value === undefined) {
+            await channel.close();
             break;
           } else {
-            await channel.push(data);
+            await channel.push(value);
           }
         }
-      });
+      } catch (exception) {
+        // Node.js stream.readable
+        values.on("readable", async () => {
+          while (true) {
+            const data = values.read();
 
-      values.once("end", channel.close);
+            if (data === null) {
+              break;
+            } else {
+              await channel.push(data);
+            }
+          }
+        });
+
+        values.once("end", channel.close);
+      }
     }
   })();
 

test/index.js

@@ -45,6 +45,12 @@ describe(`Channel`, function() {
   });
 
   describe(`from`, function() {
+    it(`callback`, async function() {
+      let counter = 0;
+      const callback = () => (counter < 3 ? counter++ : undefined);
+      assert.deepEqual(await Channel.from(callback).values(), [0, 1, 2]);
+    });
+
     it(`iterable`, async function() {
       assert.deepEqual(await Channel.from([0, 1, 2]).values(), [0, 1, 2]);
     });