diff --git a/.gitignore b/.gitignore
index ad9da58..74fbacf 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 /vendor/
 /.*/
+/composer.lock
diff --git a/.travis.yml b/.travis.yml
index 06d2664..30016b3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,19 +6,30 @@ sudo: false
 language: php
 
 php:
-  - 5.5
-  - 5.6
   - 7.0
   - 7.1
+  - 7.2
+
+env:
+  matrix:
+    -
+    - DEPENDENCIES=--prefer-lowest
+
+matrix:
+  fast_finish: true
+
+cache:
+  directories:
+    - vendor
 
 install:
   - alias composer=composer\ -n && composer selfupdate
   - composer validate
-  - composer --prefer-source install
+  - composer update --no-progress --no-suggest $DEPENDENCIES
 
 script:
-  - bin/test --coverage-clover=build/logs/clover.xml
+  - composer test -- --coverage-clover=build/logs/clover.xml
 
 after_success:
-  - composer --prefer-source require satooshi/php-coveralls
-  - vendor/bin/coveralls -v
+  - composer require php-coveralls/php-coveralls:^2
+  - vendor/bin/php-coveralls -v
diff --git a/bin/test b/bin/test
deleted file mode 100755
index 8d3beb6..0000000
--- a/bin/test
+++ /dev/null
@@ -1,5 +0,0 @@
-#!/usr/bin/env bash
-
-cd "$(dirname "$0")"/..
-
-vendor/bin/phpunit -c test "$@"
diff --git a/composer.json b/composer.json
index 387c7ac..2bb509d 100644
--- a/composer.json
+++ b/composer.json
@@ -13,10 +13,11 @@
     }
   ],
   "require": {
-    "php": "^5.5|^7"
+    "php": "^7"
   },
   "require-dev": {
-    "phpunit/phpunit": "^4"
+    "phpunit/phpunit": "^4.8",
+    "amphp/amp": "^2"
   },
   "autoload": {
     "files": [
@@ -25,5 +26,8 @@
     "psr-4": {
       "ScriptFUSION\\Retry\\": "src"
     }
+  },
+  "scripts": {
+    "test": "phpunit -c test"
   }
 }
diff --git a/composer.lock b/composer.lock
deleted file mode 100644
index fab0501..0000000
--- a/composer.lock
+++ /dev/null
@@ -1,1124 +0,0 @@
-{
-    "_readme": [
-        "This file locks the dependencies of your project to a known state",
-        "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file",
-        "This file is @generated automatically"
-    ],
-    "hash": "6c5ad455bb73996e96823decc28e47d9",
-    "content-hash": "49ce23c064f14fc0e85f490231aa73b0",
-    "packages": [],
-    "packages-dev": [
-        {
-            "name": "doctrine/instantiator",
-            "version": "1.0.5",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/doctrine/instantiator.git",
-                "reference": "8e884e78f9f0eb1329e445619e04456e64d8051d"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/doctrine/instantiator/zipball/8e884e78f9f0eb1329e445619e04456e64d8051d",
-                "reference": "8e884e78f9f0eb1329e445619e04456e64d8051d",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3,<8.0-DEV"
-            },
-            "require-dev": {
-                "athletic/athletic": "~0.1.8",
-                "ext-pdo": "*",
-                "ext-phar": "*",
-                "phpunit/phpunit": "~4.0",
-                "squizlabs/php_codesniffer": "~2.0"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.0.x-dev"
-                }
-            },
-            "autoload": {
-                "psr-4": {
-                    "Doctrine\\Instantiator\\": "src/Doctrine/Instantiator/"
-                }
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "MIT"
-            ],
-            "authors": [
-                {
-                    "name": "Marco Pivetta",
-                    "email": "ocramius@gmail.com",
-                    "homepage": "http://ocramius.github.com/"
-                }
-            ],
-            "description": "A small, lightweight utility to instantiate objects in PHP without invoking their constructors",
-            "homepage": "https://github.com/doctrine/instantiator",
-            "keywords": [
-                "constructor",
-                "instantiate"
-            ],
-            "time": "2015-06-14 21:17:01"
-        },
-        {
-            "name": "phpdocumentor/reflection-common",
-            "version": "1.0",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/phpDocumentor/ReflectionCommon.git",
-                "reference": "144c307535e82c8fdcaacbcfc1d6d8eeb896687c"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/phpDocumentor/ReflectionCommon/zipball/144c307535e82c8fdcaacbcfc1d6d8eeb896687c",
-                "reference": "144c307535e82c8fdcaacbcfc1d6d8eeb896687c",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.5"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "^4.6"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.0.x-dev"
-                }
-            },
-            "autoload": {
-                "psr-4": {
-                    "phpDocumentor\\Reflection\\": [
-                        "src"
-                    ]
-                }
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "MIT"
-            ],
-            "authors": [
-                {
-                    "name": "Jaap van Otterdijk",
-                    "email": "opensource@ijaap.nl"
-                }
-            ],
-            "description": "Common reflection classes used by phpdocumentor to reflect the code structure",
-            "homepage": "http://www.phpdoc.org",
-            "keywords": [
-                "FQSEN",
-                "phpDocumentor",
-                "phpdoc",
-                "reflection",
-                "static analysis"
-            ],
-            "time": "2015-12-27 11:43:31"
-        },
-        {
-            "name": "phpdocumentor/reflection-docblock",
-            "version": "3.1.0",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/phpDocumentor/ReflectionDocBlock.git",
-                "reference": "9270140b940ff02e58ec577c237274e92cd40cdd"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/phpDocumentor/ReflectionDocBlock/zipball/9270140b940ff02e58ec577c237274e92cd40cdd",
-                "reference": "9270140b940ff02e58ec577c237274e92cd40cdd",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.5",
-                "phpdocumentor/reflection-common": "^1.0@dev",
-                "phpdocumentor/type-resolver": "^0.2.0",
-                "webmozart/assert": "^1.0"
-            },
-            "require-dev": {
-                "mockery/mockery": "^0.9.4",
-                "phpunit/phpunit": "^4.4"
-            },
-            "type": "library",
-            "autoload": {
-                "psr-4": {
-                    "phpDocumentor\\Reflection\\": [
-                        "src/"
-                    ]
-                }
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "MIT"
-            ],
-            "authors": [
-                {
-                    "name": "Mike van Riel",
-                    "email": "me@mikevanriel.com"
-                }
-            ],
-            "description": "With this component, a library can provide support for annotations via DocBlocks or otherwise retrieve information that is embedded in a DocBlock.",
-            "time": "2016-06-10 09:48:41"
-        },
-        {
-            "name": "phpdocumentor/type-resolver",
-            "version": "0.2",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/phpDocumentor/TypeResolver.git",
-                "reference": "b39c7a5b194f9ed7bd0dd345c751007a41862443"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/phpDocumentor/TypeResolver/zipball/b39c7a5b194f9ed7bd0dd345c751007a41862443",
-                "reference": "b39c7a5b194f9ed7bd0dd345c751007a41862443",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.5",
-                "phpdocumentor/reflection-common": "^1.0"
-            },
-            "require-dev": {
-                "mockery/mockery": "^0.9.4",
-                "phpunit/phpunit": "^5.2||^4.8.24"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.0.x-dev"
-                }
-            },
-            "autoload": {
-                "psr-4": {
-                    "phpDocumentor\\Reflection\\": [
-                        "src/"
-                    ]
-                }
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "MIT"
-            ],
-            "authors": [
-                {
-                    "name": "Mike van Riel",
-                    "email": "me@mikevanriel.com"
-                }
-            ],
-            "time": "2016-06-10 07:14:17"
-        },
-        {
-            "name": "phpspec/prophecy",
-            "version": "v1.6.1",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/phpspec/prophecy.git",
-                "reference": "58a8137754bc24b25740d4281399a4a3596058e0"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/phpspec/prophecy/zipball/58a8137754bc24b25740d4281399a4a3596058e0",
-                "reference": "58a8137754bc24b25740d4281399a4a3596058e0",
-                "shasum": ""
-            },
-            "require": {
-                "doctrine/instantiator": "^1.0.2",
-                "php": "^5.3|^7.0",
-                "phpdocumentor/reflection-docblock": "^2.0|^3.0.2",
-                "sebastian/comparator": "^1.1",
-                "sebastian/recursion-context": "^1.0"
-            },
-            "require-dev": {
-                "phpspec/phpspec": "^2.0"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.6.x-dev"
-                }
-            },
-            "autoload": {
-                "psr-0": {
-                    "Prophecy\\": "src/"
-                }
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "MIT"
-            ],
-            "authors": [
-                {
-                    "name": "Konstantin Kudryashov",
-                    "email": "ever.zet@gmail.com",
-                    "homepage": "http://everzet.com"
-                },
-                {
-                    "name": "Marcello Duarte",
-                    "email": "marcello.duarte@gmail.com"
-                }
-            ],
-            "description": "Highly opinionated mocking framework for PHP 5.3+",
-            "homepage": "https://github.com/phpspec/prophecy",
-            "keywords": [
-                "Double",
-                "Dummy",
-                "fake",
-                "mock",
-                "spy",
-                "stub"
-            ],
-            "time": "2016-06-07 08:13:47"
-        },
-        {
-            "name": "phpunit/php-code-coverage",
-            "version": "2.2.4",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/php-code-coverage.git",
-                "reference": "eabf68b476ac7d0f73793aada060f1c1a9bf8979"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/php-code-coverage/zipball/eabf68b476ac7d0f73793aada060f1c1a9bf8979",
-                "reference": "eabf68b476ac7d0f73793aada060f1c1a9bf8979",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3.3",
-                "phpunit/php-file-iterator": "~1.3",
-                "phpunit/php-text-template": "~1.2",
-                "phpunit/php-token-stream": "~1.3",
-                "sebastian/environment": "^1.3.2",
-                "sebastian/version": "~1.0"
-            },
-            "require-dev": {
-                "ext-xdebug": ">=2.1.4",
-                "phpunit/phpunit": "~4"
-            },
-            "suggest": {
-                "ext-dom": "*",
-                "ext-xdebug": ">=2.2.1",
-                "ext-xmlwriter": "*"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "2.2.x-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sb@sebastian-bergmann.de",
-                    "role": "lead"
-                }
-            ],
-            "description": "Library that provides collection, processing, and rendering functionality for PHP code coverage information.",
-            "homepage": "https://github.com/sebastianbergmann/php-code-coverage",
-            "keywords": [
-                "coverage",
-                "testing",
-                "xunit"
-            ],
-            "time": "2015-10-06 15:47:00"
-        },
-        {
-            "name": "phpunit/php-file-iterator",
-            "version": "1.4.1",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/php-file-iterator.git",
-                "reference": "6150bf2c35d3fc379e50c7602b75caceaa39dbf0"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/php-file-iterator/zipball/6150bf2c35d3fc379e50c7602b75caceaa39dbf0",
-                "reference": "6150bf2c35d3fc379e50c7602b75caceaa39dbf0",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3.3"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.4.x-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sb@sebastian-bergmann.de",
-                    "role": "lead"
-                }
-            ],
-            "description": "FilterIterator implementation that filters files based on a list of suffixes.",
-            "homepage": "https://github.com/sebastianbergmann/php-file-iterator/",
-            "keywords": [
-                "filesystem",
-                "iterator"
-            ],
-            "time": "2015-06-21 13:08:43"
-        },
-        {
-            "name": "phpunit/php-text-template",
-            "version": "1.2.1",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/php-text-template.git",
-                "reference": "31f8b717e51d9a2afca6c9f046f5d69fc27c8686"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/php-text-template/zipball/31f8b717e51d9a2afca6c9f046f5d69fc27c8686",
-                "reference": "31f8b717e51d9a2afca6c9f046f5d69fc27c8686",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3.3"
-            },
-            "type": "library",
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de",
-                    "role": "lead"
-                }
-            ],
-            "description": "Simple template engine.",
-            "homepage": "https://github.com/sebastianbergmann/php-text-template/",
-            "keywords": [
-                "template"
-            ],
-            "time": "2015-06-21 13:50:34"
-        },
-        {
-            "name": "phpunit/php-timer",
-            "version": "1.0.8",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/php-timer.git",
-                "reference": "38e9124049cf1a164f1e4537caf19c99bf1eb260"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/php-timer/zipball/38e9124049cf1a164f1e4537caf19c99bf1eb260",
-                "reference": "38e9124049cf1a164f1e4537caf19c99bf1eb260",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3.3"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "~4|~5"
-            },
-            "type": "library",
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sb@sebastian-bergmann.de",
-                    "role": "lead"
-                }
-            ],
-            "description": "Utility class for timing",
-            "homepage": "https://github.com/sebastianbergmann/php-timer/",
-            "keywords": [
-                "timer"
-            ],
-            "time": "2016-05-12 18:03:57"
-        },
-        {
-            "name": "phpunit/php-token-stream",
-            "version": "1.4.8",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/php-token-stream.git",
-                "reference": "3144ae21711fb6cac0b1ab4cbe63b75ce3d4e8da"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/php-token-stream/zipball/3144ae21711fb6cac0b1ab4cbe63b75ce3d4e8da",
-                "reference": "3144ae21711fb6cac0b1ab4cbe63b75ce3d4e8da",
-                "shasum": ""
-            },
-            "require": {
-                "ext-tokenizer": "*",
-                "php": ">=5.3.3"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "~4.2"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.4-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de"
-                }
-            ],
-            "description": "Wrapper around PHP's tokenizer extension.",
-            "homepage": "https://github.com/sebastianbergmann/php-token-stream/",
-            "keywords": [
-                "tokenizer"
-            ],
-            "time": "2015-09-15 10:49:45"
-        },
-        {
-            "name": "phpunit/phpunit",
-            "version": "4.8.26",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/phpunit.git",
-                "reference": "fc1d8cd5b5de11625979125c5639347896ac2c74"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/phpunit/zipball/fc1d8cd5b5de11625979125c5639347896ac2c74",
-                "reference": "fc1d8cd5b5de11625979125c5639347896ac2c74",
-                "shasum": ""
-            },
-            "require": {
-                "ext-dom": "*",
-                "ext-json": "*",
-                "ext-pcre": "*",
-                "ext-reflection": "*",
-                "ext-spl": "*",
-                "php": ">=5.3.3",
-                "phpspec/prophecy": "^1.3.1",
-                "phpunit/php-code-coverage": "~2.1",
-                "phpunit/php-file-iterator": "~1.4",
-                "phpunit/php-text-template": "~1.2",
-                "phpunit/php-timer": "^1.0.6",
-                "phpunit/phpunit-mock-objects": "~2.3",
-                "sebastian/comparator": "~1.1",
-                "sebastian/diff": "~1.2",
-                "sebastian/environment": "~1.3",
-                "sebastian/exporter": "~1.2",
-                "sebastian/global-state": "~1.0",
-                "sebastian/version": "~1.0",
-                "symfony/yaml": "~2.1|~3.0"
-            },
-            "suggest": {
-                "phpunit/php-invoker": "~1.1"
-            },
-            "bin": [
-                "phpunit"
-            ],
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "4.8.x-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de",
-                    "role": "lead"
-                }
-            ],
-            "description": "The PHP Unit Testing framework.",
-            "homepage": "https://phpunit.de/",
-            "keywords": [
-                "phpunit",
-                "testing",
-                "xunit"
-            ],
-            "time": "2016-05-17 03:09:28"
-        },
-        {
-            "name": "phpunit/phpunit-mock-objects",
-            "version": "2.3.8",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/phpunit-mock-objects.git",
-                "reference": "ac8e7a3db35738d56ee9a76e78a4e03d97628983"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/phpunit-mock-objects/zipball/ac8e7a3db35738d56ee9a76e78a4e03d97628983",
-                "reference": "ac8e7a3db35738d56ee9a76e78a4e03d97628983",
-                "shasum": ""
-            },
-            "require": {
-                "doctrine/instantiator": "^1.0.2",
-                "php": ">=5.3.3",
-                "phpunit/php-text-template": "~1.2",
-                "sebastian/exporter": "~1.2"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "~4.4"
-            },
-            "suggest": {
-                "ext-soap": "*"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "2.3.x-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sb@sebastian-bergmann.de",
-                    "role": "lead"
-                }
-            ],
-            "description": "Mock Object library for PHPUnit",
-            "homepage": "https://github.com/sebastianbergmann/phpunit-mock-objects/",
-            "keywords": [
-                "mock",
-                "xunit"
-            ],
-            "time": "2015-10-02 06:51:40"
-        },
-        {
-            "name": "sebastian/comparator",
-            "version": "1.2.0",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/comparator.git",
-                "reference": "937efb279bd37a375bcadf584dec0726f84dbf22"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/comparator/zipball/937efb279bd37a375bcadf584dec0726f84dbf22",
-                "reference": "937efb279bd37a375bcadf584dec0726f84dbf22",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3.3",
-                "sebastian/diff": "~1.2",
-                "sebastian/exporter": "~1.2"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "~4.4"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.2.x-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Jeff Welch",
-                    "email": "whatthejeff@gmail.com"
-                },
-                {
-                    "name": "Volker Dusch",
-                    "email": "github@wallbash.com"
-                },
-                {
-                    "name": "Bernhard Schussek",
-                    "email": "bschussek@2bepublished.at"
-                },
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de"
-                }
-            ],
-            "description": "Provides the functionality to compare PHP values for equality",
-            "homepage": "http://www.github.com/sebastianbergmann/comparator",
-            "keywords": [
-                "comparator",
-                "compare",
-                "equality"
-            ],
-            "time": "2015-07-26 15:48:44"
-        },
-        {
-            "name": "sebastian/diff",
-            "version": "1.4.1",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/diff.git",
-                "reference": "13edfd8706462032c2f52b4b862974dd46b71c9e"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/diff/zipball/13edfd8706462032c2f52b4b862974dd46b71c9e",
-                "reference": "13edfd8706462032c2f52b4b862974dd46b71c9e",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3.3"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "~4.8"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.4-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Kore Nordmann",
-                    "email": "mail@kore-nordmann.de"
-                },
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de"
-                }
-            ],
-            "description": "Diff implementation",
-            "homepage": "https://github.com/sebastianbergmann/diff",
-            "keywords": [
-                "diff"
-            ],
-            "time": "2015-12-08 07:14:41"
-        },
-        {
-            "name": "sebastian/environment",
-            "version": "1.3.7",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/environment.git",
-                "reference": "4e8f0da10ac5802913afc151413bc8c53b6c2716"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/environment/zipball/4e8f0da10ac5802913afc151413bc8c53b6c2716",
-                "reference": "4e8f0da10ac5802913afc151413bc8c53b6c2716",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3.3"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "~4.4"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.3.x-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de"
-                }
-            ],
-            "description": "Provides functionality to handle HHVM/PHP environments",
-            "homepage": "http://www.github.com/sebastianbergmann/environment",
-            "keywords": [
-                "Xdebug",
-                "environment",
-                "hhvm"
-            ],
-            "time": "2016-05-17 03:18:57"
-        },
-        {
-            "name": "sebastian/exporter",
-            "version": "1.2.2",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/exporter.git",
-                "reference": "42c4c2eec485ee3e159ec9884f95b431287edde4"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/exporter/zipball/42c4c2eec485ee3e159ec9884f95b431287edde4",
-                "reference": "42c4c2eec485ee3e159ec9884f95b431287edde4",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3.3",
-                "sebastian/recursion-context": "~1.0"
-            },
-            "require-dev": {
-                "ext-mbstring": "*",
-                "phpunit/phpunit": "~4.4"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.3.x-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Jeff Welch",
-                    "email": "whatthejeff@gmail.com"
-                },
-                {
-                    "name": "Volker Dusch",
-                    "email": "github@wallbash.com"
-                },
-                {
-                    "name": "Bernhard Schussek",
-                    "email": "bschussek@2bepublished.at"
-                },
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de"
-                },
-                {
-                    "name": "Adam Harvey",
-                    "email": "aharvey@php.net"
-                }
-            ],
-            "description": "Provides the functionality to export PHP variables for visualization",
-            "homepage": "http://www.github.com/sebastianbergmann/exporter",
-            "keywords": [
-                "export",
-                "exporter"
-            ],
-            "time": "2016-06-17 09:04:28"
-        },
-        {
-            "name": "sebastian/global-state",
-            "version": "1.1.1",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/global-state.git",
-                "reference": "bc37d50fea7d017d3d340f230811c9f1d7280af4"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/global-state/zipball/bc37d50fea7d017d3d340f230811c9f1d7280af4",
-                "reference": "bc37d50fea7d017d3d340f230811c9f1d7280af4",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3.3"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "~4.2"
-            },
-            "suggest": {
-                "ext-uopz": "*"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.0-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de"
-                }
-            ],
-            "description": "Snapshotting of global state",
-            "homepage": "http://www.github.com/sebastianbergmann/global-state",
-            "keywords": [
-                "global state"
-            ],
-            "time": "2015-10-12 03:26:01"
-        },
-        {
-            "name": "sebastian/recursion-context",
-            "version": "1.0.2",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/recursion-context.git",
-                "reference": "913401df809e99e4f47b27cdd781f4a258d58791"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/recursion-context/zipball/913401df809e99e4f47b27cdd781f4a258d58791",
-                "reference": "913401df809e99e4f47b27cdd781f4a258d58791",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3.3"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "~4.4"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.0.x-dev"
-                }
-            },
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Jeff Welch",
-                    "email": "whatthejeff@gmail.com"
-                },
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de"
-                },
-                {
-                    "name": "Adam Harvey",
-                    "email": "aharvey@php.net"
-                }
-            ],
-            "description": "Provides functionality to recursively process PHP variables",
-            "homepage": "http://www.github.com/sebastianbergmann/recursion-context",
-            "time": "2015-11-11 19:50:13"
-        },
-        {
-            "name": "sebastian/version",
-            "version": "1.0.6",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/sebastianbergmann/version.git",
-                "reference": "58b3a85e7999757d6ad81c787a1fbf5ff6c628c6"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/sebastianbergmann/version/zipball/58b3a85e7999757d6ad81c787a1fbf5ff6c628c6",
-                "reference": "58b3a85e7999757d6ad81c787a1fbf5ff6c628c6",
-                "shasum": ""
-            },
-            "type": "library",
-            "autoload": {
-                "classmap": [
-                    "src/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "BSD-3-Clause"
-            ],
-            "authors": [
-                {
-                    "name": "Sebastian Bergmann",
-                    "email": "sebastian@phpunit.de",
-                    "role": "lead"
-                }
-            ],
-            "description": "Library that helps with managing the version number of Git-hosted PHP projects",
-            "homepage": "https://github.com/sebastianbergmann/version",
-            "time": "2015-06-21 13:59:46"
-        },
-        {
-            "name": "symfony/yaml",
-            "version": "v3.1.2",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/symfony/yaml.git",
-                "reference": "2884c26ce4c1d61aebf423a8b912950fe7c764de"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/symfony/yaml/zipball/2884c26ce4c1d61aebf423a8b912950fe7c764de",
-                "reference": "2884c26ce4c1d61aebf423a8b912950fe7c764de",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.5.9"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "3.1-dev"
-                }
-            },
-            "autoload": {
-                "psr-4": {
-                    "Symfony\\Component\\Yaml\\": ""
-                },
-                "exclude-from-classmap": [
-                    "/Tests/"
-                ]
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "MIT"
-            ],
-            "authors": [
-                {
-                    "name": "Fabien Potencier",
-                    "email": "fabien@symfony.com"
-                },
-                {
-                    "name": "Symfony Community",
-                    "homepage": "https://symfony.com/contributors"
-                }
-            ],
-            "description": "Symfony Yaml Component",
-            "homepage": "https://symfony.com",
-            "time": "2016-06-29 05:41:56"
-        },
-        {
-            "name": "webmozart/assert",
-            "version": "1.0.2",
-            "source": {
-                "type": "git",
-                "url": "https://github.com/webmozart/assert.git",
-                "reference": "30eed06dd6bc88410a4ff7f77b6d22f3ce13dbde"
-            },
-            "dist": {
-                "type": "zip",
-                "url": "https://api.github.com/repos/webmozart/assert/zipball/30eed06dd6bc88410a4ff7f77b6d22f3ce13dbde",
-                "reference": "30eed06dd6bc88410a4ff7f77b6d22f3ce13dbde",
-                "shasum": ""
-            },
-            "require": {
-                "php": ">=5.3.3"
-            },
-            "require-dev": {
-                "phpunit/phpunit": "^4.6"
-            },
-            "type": "library",
-            "extra": {
-                "branch-alias": {
-                    "dev-master": "1.0-dev"
-                }
-            },
-            "autoload": {
-                "psr-4": {
-                    "Webmozart\\Assert\\": "src/"
-                }
-            },
-            "notification-url": "https://packagist.org/downloads/",
-            "license": [
-                "MIT"
-            ],
-            "authors": [
-                {
-                    "name": "Bernhard Schussek",
-                    "email": "bschussek@gmail.com"
-                }
-            ],
-            "description": "Assertions to validate method input/output with nice error messages.",
-            "keywords": [
-                "assert",
-                "check",
-                "validate"
-            ],
-            "time": "2015-08-24 13:29:44"
-        }
-    ],
-    "aliases": [],
-    "minimum-stability": "stable",
-    "stability-flags": [],
-    "prefer-stable": false,
-    "prefer-lowest": false,
-    "platform": {
-        "php": "^5.5|^7"
-    },
-    "platform-dev": []
-}
diff --git a/src/retry.php b/src/retry.php
index e1a9e60..8345a62 100644
--- a/src/retry.php
+++ b/src/retry.php
@@ -1,34 +1,85 @@
 <?php
 namespace ScriptFUSION\Retry;
 
+use Amp\Coroutine;
+use Amp\Promise;
+use Amp\Success;
+
 /**
  * Tries the specified operation up to the specified number of times. If specified, the exception handler will be
- * called immediately before retrying the operation.
+ * called immediately before retrying the operation. If the error handler returns false, the operation will not be
+ * retried.
  *
  * @param int $tries Number of times.
  * @param callable $operation Operation.
  * @param callable $onError Optional. Exception handler.
  *
  * @return mixed Result of running the operation if tries is greater than zero, otherwise null.
+ *
+ * @throws FailingTooHardException The maximum number of attempts was reached.
+ * @throws \UnexpectedValueException The operation returned an unsupported type.
  */
 function retry($tries, callable $operation, callable $onError = null)
 {
-    if (($tries |= 0) <= $attempts = 0) {
-        return;
-    }
+    /** @var \Generator $generator */
+    $generator = (static function () use ($tries, $operation, $onError) {
+        if (($tries |= 0) <= $attempts = 0) {
+            return;
+        }
+
+        try {
+            beginning:
 
-    try {
-        beginning:
-        return $operation();
-    } catch (\Exception $exception) {
-        if ($tries === ++$attempts) {
-            throw new FailingTooHardException($attempts, $exception);
+            if (($result = $operation()) instanceof Promise) {
+                // Wait for promise to resolve.
+                $result = yield $result;
+            }
+        } catch (\Exception $exception) {
+            if ($tries === ++$attempts) {
+                throw new FailingTooHardException($attempts, $exception);
+            }
+
+            if ($onError && $onError($exception) === false) {
+                return;
+            }
+
+            goto beginning;
         }
 
-        if ($onError && $onError($exception) === false) {
-            return;
+        if ($result instanceof \Generator) {
+            throw new \UnexpectedValueException('Cannot retry a Generator. You probably meant something else.');
         }
 
-        goto beginning;
+        return $result;
+    })();
+
+    // Normal code path: generator runs without yielding.
+    if (!$generator->valid()) {
+        return $generator->getReturn();
     }
+
+    // Async code path: generator yields promises.
+    return $generator;
+}
+
+/**
+ * Tries the specified operation up to the specified number of times. If specified, the exception handler will be
+ * called immediately before retrying the operation. If the error handler returns false, the operation will not be
+ * retried.
+ *
+ * @param int $tries Number of times.
+ * @param callable $operation Operation.
+ * @param callable $onError Optional. Exception handler.
+ *
+ * @return Promise Promise that returns the result of running the operation if tries is greater than zero, otherwise
+ *     a promise that yields null.
+ *
+ * @throws FailingTooHardException The maximum number of attempts was reached.
+ * @throws \UnexpectedValueException The operation returned an unsupported type.
+ */
+function retryAsync($tries, callable $operation, callable $onError = null): Promise
+{
+    $generator = retry($tries, $operation, $onError);
+
+    return $generator !== null ? new Coroutine($generator) : new Success(null);
 }
diff --git a/test/RetryAsyncTest.php b/test/RetryAsyncTest.php
new file mode 100644
index 0000000..a75409f
--- /dev/null
+++ b/test/RetryAsyncTest.php
@@ -0,0 +1,139 @@
+<?php
+namespace ScriptFUSIONTest\Retry;
+
+use Amp\Delayed;
+use ScriptFUSION\Retry\FailingTooHardException;
+
+final class RetryAsyncTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * Tests that a successful promise is returned without retrying.
+     */
+    public function testWithoutFailingAsync()
+    {
+        $invocations = 0;
+
+        $value = \Amp\Promise\wait(
+            \ScriptFUSION\Retry\retryAsync($tries = 1, static function () use (&$invocations) {
+                ++$invocations;
+
+                return new Delayed(0, 'foo');
+            })
+        );
+
+        self::assertSame($tries, $invocations);
+        self::assertSame('foo', $value);
+    }
+
+    /**
+     * Tests that a failed promise is retried.
+     */
+    public function testFailingOnceAsync()
+    {
+        $invocations = 0;
+        $failed = false;
+
+        $value = \Amp\Promise\wait(
+            \ScriptFUSION\Retry\retryAsync($tries = 2, static function () use (&$invocations, &$failed) {
+                ++$invocations;
+
+                if (!$failed) {
+                    $failed = true;
+
+                    throw new \RuntimeException;
+                }
+
+                return new Delayed(0, 'foo');
+            })
+        );
+
+        self::assertTrue($failed);
+        self::assertSame($tries, $invocations);
+        self::assertSame('foo', $value);
+    }
+
+    /**
+     * Tests that trying zero times yields null.
+     */
+    public function testZeroTriesAsync()
+    {
+        $invocations = 0;
+
+        $value = \Amp\Promise\wait(
+            \ScriptFUSION\Retry\retryAsync($tries = 0, static function () use (&$invocations) {
+                ++$invocations;
+
+                return new Delayed(0, 'foo');
+            })
+        );
+
+        self::assertSame($tries, $invocations);
+        self::assertNull($value);
+    }
+
+    /**
+     * Tests that reaching maximum tries throws FailingTooHardException.
+     */
+    public function testFailingTooHardAsync()
+    {
+        $invocations = 0;
+        $outerException = $innerException = null;
+
+        try {
+            \ScriptFUSION\Retry\retryAsync($tries = 3, static function () use (&$invocations, &$innerException) {
+                ++$invocations;
+
+                throw $innerException = new \RuntimeException;
+            });
+        } catch (FailingTooHardException $outerException) {
+        }
+
+        self::assertInstanceOf(FailingTooHardException::class, $outerException);
+        self::assertSame($innerException, $outerException->getPrevious());
+        self::assertSame($tries, $invocations);
+    }
+
+    /**
+     * These that the error callback is called before each retry.
+     */
+    public function testErrorCallbackAsync()
+    {
+        $invocations = $errors = 0;
+        $outerException = $innerException = null;
+
+        try {
+            \ScriptFUSION\Retry\retryAsync($tries = 3, static function () use (&$invocations, &$innerException) {
+                ++$invocations;
+
+                throw $innerException = new \RuntimeException;
+            }, function (\Exception $exception) use (&$innerException, &$errors) {
+                ++$errors;
+
+                self::assertSame($innerException, $exception);
+            });
+        } catch (FailingTooHardException $outerException) {
+        }
+
+        self::assertInstanceOf(FailingTooHardException::class, $outerException);
+        self::assertSame($tries, $invocations);
+        self::assertSame($tries - 1, $errors);
+    }
+
+    /**
+     * Tests that an error handler that returns false aborts retrying.
+     */
+    public function testErrorCallbackHaltAsync()
+    {
+        $invocations = 0;
+
+        \ScriptFUSION\Retry\retryAsync($tries = 2, static function () use (&$invocations) {
+            ++$invocations;
+
+            throw new \RuntimeException;
+        }, static function () {
+            return false;
+        });
+
+        self::assertSame(1, $invocations);
+    }
+}
diff --git a/test/RetryTest.php b/test/RetryTest.php
index b780977..49f3f13 100644
--- a/test/RetryTest.php
+++ b/test/RetryTest.php
@@ -9,7 +9,7 @@ public function testWithoutFailing()
     {
         $invocations = 0;
 
-        $value = \ScriptFUSION\Retry\retry($tries = 1, function () use (&$invocations) {
+        $value = \ScriptFUSION\Retry\retry($tries = 1, static function () use (&$invocations) {
             ++$invocations;
 
             return 5;
@@ -24,7 +24,7 @@ public function testFailingOnce()
         $invocations = 0;
         $failed = false;
 
-        $value = \ScriptFUSION\Retry\retry($tries = 2, function () use (&$invocations, &$failed) {
+        $value = \ScriptFUSION\Retry\retry($tries = 2, static function () use (&$invocations, &$failed) {
             ++$invocations;
 
             if (!$failed) {
@@ -45,7 +45,7 @@ public function testZeroTries()
     {
         $invocations = 0;
 
-        $value = \ScriptFUSION\Retry\retry($tries = 0, function () use (&$invocations) {
+        $value = \ScriptFUSION\Retry\retry($tries = 0, static function () use (&$invocations) {
             ++$invocations;
 
             return 5;
@@ -61,7 +61,7 @@ public function testFailingTooHard()
         $outerException = $innerException = null;
 
         try {
-            \ScriptFUSION\Retry\retry($tries = 2, function () use (&$invocations, &$innerException) {
+            \ScriptFUSION\Retry\retry($tries = 2, static function () use (&$invocations, &$innerException) {
                 ++$invocations;
 
                 throw $innerException = new \RuntimeException;
@@ -80,7 +80,7 @@ public function testErrorCallback()
         $outerException = $innerException = null;
 
         try {
-            \ScriptFUSION\Retry\retry($tries = 2, function () use (&$invocations, &$innerException) {
+            \ScriptFUSION\Retry\retry($tries = 2, static function () use (&$invocations, &$innerException) {
                 ++$invocations;
 
                 throw $innerException = new \RuntimeException;
@@ -97,20 +97,20 @@ public function testErrorCallback()
         self::assertSame($tries - 1, $errors);
     }
 
+    /**
+     * Tests that an error handler that returns false aborts retrying.
+     */
     public function testErrorCallbackHalt()
     {
         $invocations = 0;
 
-        try {
-            \ScriptFUSION\Retry\retry($tries = 2, function () use (&$invocations) {
-                ++$invocations;
+        \ScriptFUSION\Retry\retry($tries = 2, static function () use (&$invocations) {
+            ++$invocations;
 
-                throw new \RuntimeException;
-            }, function () {
-                return false;
-            });
-        } catch (FailingTooHardException $exception) {
-        }
+            throw new \RuntimeException;
+        }, static function () {
+            return false;
+        });
 
         self::assertSame(1, $invocations);
     }