add asynchronous implementation

README.md

@@ -2,8 +2,10 @@
 
 read from one pull-stream, then the next, then the next...
 
-when one stream end (unless it errored) call a function
-to get the next stream. much like [pull-cat](https://github.com/pull-stream/pull-cat)
+when one stream end (unless it errored) call a function to get the next
+stream.  there is also a function that takes an asynchronous function.
+
+much like [pull-cat](https://github.com/pull-stream/pull-cat)
 except creates streams by calling a function instead of takeing them out of an array.
 
 in particular, this is useful for making a read stream that reconnects

async.js

@@ -0,0 +1,36 @@
+var noop = function () {}
+
+module.exports = function (next) {
+  var stream
+  return function (abort, cb) {
+    if(!cb) throw new Error('callback required!')
+    if(abort) {
+      if(stream) stream(abort, cb)
+      else       cb(abort)
+    }
+    else more()
+
+    function more () {
+      if(stream) send()
+      else {
+        next(function (error, nextStream) {
+          if(error) return cb(error)
+          if(!nextStream) return cb(true)
+          stream = nextStream
+          send()
+        })
+      }
+    }
+
+    function send () {
+      stream(null, function (err, data) {
+        if(err) {
+          stream = null
+          if(err === true) setTimeout(more, 100)
+          else             cb(err)
+        }
+        else cb(null, data)
+      })
+    }
+  }
+}

package.json

@@ -8,7 +8,9 @@
     "url": "git://github.com/dominictarr/pull-next.git"
   },
   "dependencies": {},
-  "devDependencies": {},
+  "devDependencies": {
+    "pull-stream": "^3.4.5"
+  },
   "scripts": {
     "test": "set -e; for t in test/*.js; do node $t; done"
   },

test/async.js

@@ -0,0 +1,23 @@
+var assert = require('assert')
+var pull = require('pull-stream')
+var asyncNext = require('../async')
+
+var arrays = [
+  ['a', 'b', 'c'],
+  ['d', 'e', 'f']
+]
+
+pull(
+  asyncNext(function (cb) {
+    if (arrays.length === 0) cb(null, false)
+    else cb(null, pull.values(arrays.shift()))
+  }),
+  pull.collect(function (err, data) {
+    assert.ifError(err, 'no error')
+    assert.deepEqual(
+      data,
+      ['a', 'b', 'c', 'd', 'e', 'f'],
+      'pulls all data'
+    )
+  })
+)