When a number is passed as concurrent parameter, execute the tasks in…

index.js

@@ -1,4 +1,5 @@
 'use strict';
+const pMap = require('p-map');
 const Task = require('./lib/task');
 const TaskWrapper = require('./lib/task-wrapper');
 const renderer = require('./lib/renderer');
@@ -79,17 +80,20 @@ class Listr {
 
 		this._checkAll(context);
 
-		let tasks;
-		if (this._options.concurrent === true) {
-			tasks = Promise.all(this._tasks.map(task => runTask(task, context, errors)));
+		let concurrency;
+		if (!this._options.concurrent) {
+			concurrency = 1;
+		} else if (typeof this._options.concurrent === 'number') {
+			concurrency = this._options.concurrent;
 		} else {
-			tasks = this._tasks.reduce((promise, task) => promise.then(() => {
-				this._checkAll(context);
-
-				return runTask(task, context, errors);
-			}), Promise.resolve());
+			concurrency = 'Infinity';
 		}
 
+		const tasks = pMap(this._tasks, task => {
+			this._checkAll(context);
+			return runTask(task, context, errors);
+		}, {concurrency});
+
 		return tasks
 			.then(() => {
 				if (errors.length > 0) {

package.json

@@ -51,6 +51,7 @@
     "log-symbols": "^1.0.2",
     "log-update": "^1.0.2",
     "ora": "^0.2.3",
+    "p-map": "^1.1.1",
     "rxjs": "^5.0.0-beta.11",
     "stream-to-observable": "^0.1.0",
     "strip-ansi": "^3.0.1"

test/concurrent.js

@@ -0,0 +1,84 @@
+import {serial as test} from 'ava';
+import delay from 'delay';
+import Listr from '../';
+import SimpleRenderer from './fixtures/simple-renderer';
+import {testOutput} from './fixtures/utils';
+
+const tasks = [
+	{
+		title: 'P1',
+		task: () => delay(100)
+	},
+	{
+		title: 'P2',
+		task: () => delay(200)
+	},
+	{
+		title: 'P3',
+		task: () => delay(300)
+	},
+	{
+		title: 'P4',
+		task: () => delay(400)
+	}
+];
+
+test('run tasks sequentially if concurrency is undefined', async t => {
+	const list = new Listr(tasks, {
+		renderer: SimpleRenderer
+	});
+
+	testOutput(t, [
+		'P1 [started]',
+		'P1 [completed]',
+		'P2 [started]',
+		'P2 [completed]',
+		'P3 [started]',
+		'P3 [completed]',
+		'P4 [started]',
+		'P4 [completed]',
+		'done'
+	]);
+
+	await list.run();
+});
+
+test('run tasks in parallel if concurrency is true', async t => {
+	const list = new Listr(tasks, {
+		renderer: SimpleRenderer,
+		concurrent: true
+	});
+
+	testOutput(t, [
+		'P1 [started]',
+		'P2 [started]',
+		'P3 [started]',
+		'P4 [started]',
+		'P1 [completed]',
+		'P2 [completed]',
+		'P3 [completed]',
+		'P4 [completed]',
+		'done'
+	]);
+
+	await list.run();
+});
+
+test('run tasks in sequential parallel chunks, if concurrency is a number', async t => {
+	const list = new Listr(tasks, {
+		renderer: SimpleRenderer,
+		concurrent: 2
+	});
+	testOutput(t, [
+		'P1 [started]',
+		'P2 [started]',
+		'P1 [completed]',
+		'P3 [started]',
+		'P2 [completed]',
+		'P4 [started]',
+		'P3 [completed]',
+		'P4 [completed]',
+		'done'
+	]);
+	await list.run();
+});