from: Support 'mapfn'.

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(callback | iterable | stream.Readable) -> Channel](#channelfromcallback--iterable--streamreadable---channel)
+    - [Channel.from(callback | iterable | stream.Readable[, mapfn [, thisArg]]) -> Channel](#channelfromcallback--iterable--streamreadable-mapfn--thisarg---channel)
   - [Channel Object](#channel-object)
     - [every(callbackfn[, thisArg]) -> async Boolean](#everycallbackfn-thisarg---async-boolean)
     - [filter(callbackfn[, thisArg]) -> Channel](#filtercallbackfn-thisarg---channel)
@@ -164,7 +164,7 @@ Return `true` if `value` is a channel, `false` otherwise.
 
 Push `values` into a new channel and then close it.
 
-### Channel.from(callback | iterable | stream.Readable) -> Channel
+### Channel.from(callback | iterable | stream.Readable[, mapfn [, thisArg]]) -> Channel
 
 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).
@@ -173,6 +173,9 @@ 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`.
 
+If the optional `mapfn` argument is provided, call it (using the also optional
+`thisArg`) on each value before pushing it into the channel.
+
 ## Channel Object
 
 ### every(callbackfn[, thisArg]) -> async Boolean

lib/index.js

@@ -306,7 +306,7 @@ const Channel = function(length = 0) {
   );
 };
 
-Channel.from = values => {
+Channel.from = (values, mapfn, thisArg) => {
   const channel = Channel();
 
   (async () => {
@@ -349,7 +349,7 @@ Channel.from = values => {
     }
   })();
 
-  return channel;
+  return mapfn ? channel.map(mapfn, thisArg) : channel;
 };
 
 Channel.of = (...values) => Channel.from(values);

test/index.js

@@ -55,6 +55,15 @@ describe(`Channel`, function() {
       assert.deepEqual(await Channel.from([0, 1, 2]).values(), [0, 1, 2]);
     });
 
+    it(`mapfn`, async function() {
+      assert.deepEqual(
+        await Channel.from([`a`, `b`, `c`], value =>
+          value.toUpperCase()
+        ).values(),
+        [`A`, `B`, `C`]
+      );
+    });
+
     it(`Node.js's stream.readOnly`, async function() {
       const readOnly = stream.PassThrough({ objectMode: true });
       readOnly.write(0);