diff --git a/composer.json b/composer.json
index d6aec63..1ba811b 100644
--- a/composer.json
+++ b/composer.json
@@ -21,12 +21,11 @@
     "minimum-stability": "dev",
     "prefer-stable": true,
     "require": {
-        "php": ">=5.5",
         "amphp/amp": "dev-master as 2.0"
     },
     "require-dev": {
         "amphp/loop": "dev-master",
-        "phpunit/phpunit": "^4|^5",
+        "phpunit/phpunit": "^5",
         "friendsofphp/php-cs-fixer": "~1.9"
     },
     "autoload": {
diff --git a/lib/Buffer.php b/lib/Buffer.php
index 62e3530..42fa545 100644
--- a/lib/Buffer.php
+++ b/lib/Buffer.php
@@ -14,8 +14,8 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      *
      * @param string $data
      */
-    public function __construct($data = '') {
-        $this->data = (string) $data;
+    public function __construct(string $data = '') {
+        $this->data = $data;
     }
     
     /**
@@ -23,14 +23,14 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      *
      * @return int
      */
-    public function getLength() {
+    public function getLength(): int {
         return \strlen($this->data);
     }
     
     /**
      * @return int
      */
-    public function count() {
+    public function count(): int {
         return $this->getLength();
     }
     
@@ -39,7 +39,7 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      *
      * @return bool
      */
-    public function isEmpty() {
+    public function isEmpty(): bool {
         return $this->data === '';
     }
     
@@ -48,7 +48,7 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      *
      * @param string $data
      */
-    public function push($data) {
+    public function push(string $data) {
         $this->data .= $data;
     }
     
@@ -57,7 +57,7 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      *
      * @param string $data
      */
-    public function unshift($data) {
+    public function unshift(string $data): string {
         $this->data = $data . $this->data;
     }
     
@@ -66,8 +66,7 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      *
      * @return string
      */
-    public function shift($length) {
-        $length = (int) $length;
+    public function shift(int $length): string {
         if ($length <= 0) {
             return '';
         }
@@ -92,13 +91,11 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      *
      * @return string
      */
-    public function peek($length, $offset = 0) {
-        $length = (int) $length;
+    public function peek(int $length, int $offset = 0): string {
         if ($length <= 0) {
             return '';
         }
         
-        $offset = (int) $offset;
         if ($offset < 0) {
             $offset = 0;
         }
@@ -115,8 +112,7 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      *
      * @return string
      */
-    public function pop($length) {
-        $length = (int) $length;
+    public function pop(int $length): string {
         if ($length <= 0) {
             return '';
         }
@@ -136,13 +132,11 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      *
      * @return string
      */
-    public function remove($length, $offset = 0) {
-        $length = (int) $length;
+    public function remove(int $length, int $offset = 0): string {
         if ($length <= 0) {
             return '';
         }
         
-        $offset = (int) $offset;
         if ($offset < 0) {
             $offset = 0;
         }
@@ -163,7 +157,7 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      *
      * @return string
      */
-    public function drain() {
+    public function drain(): string {
         $buffer = $this->data;
         $this->data = '';
         return $buffer;
@@ -175,7 +169,7 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      * @param string $string
      * @param int $position
      */
-    public function insert($string, $position) {
+    public function insert(string $string, int  $position) {
         $this->data = \substr_replace($this->data, $string, $position, 0);
     }
     
@@ -187,7 +181,7 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      *
      * @return int Number of replacements performed.
      */
-    public function replace($search, $replace) {
+    public function replace($search, $replace): int {
         $this->data = \str_replace($search, $replace, $this->data, $count);
         return $count;
     }
@@ -202,7 +196,7 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
      *
      * @see strpos()
      */
-    public function search($string, $reverse = false) {
+    public function search(string $string, bool $reverse = false) {
         if ($reverse) {
             return \strrpos($this->data, $string);
         }
@@ -256,14 +250,14 @@ class Buffer implements \ArrayAccess, \Countable, \IteratorAggregate
     /**
      * @return BufferIterator
      */
-    public function getIterator() {
+    public function getIterator(): \Iterator {
         return new BufferIterator($this);
     }
     
     /**
      * @return string
      */
-    public function __toString() {
+    public function __toString(): string {
         return $this->data;
     }
 }
diff --git a/lib/BufferIterator.php b/lib/BufferIterator.php
index feb67b0..08d89c3 100644
--- a/lib/BufferIterator.php
+++ b/lib/BufferIterator.php
@@ -6,14 +6,10 @@ namespace Amp\Stream;
  */
 class BufferIterator implements \SeekableIterator
 {
-    /**
-     * @var Buffer
-     */
+    /** @var \Amp\Stream\Buffer */
     private $buffer;
     
-    /**
-     * @var int
-     */
+    /** @var int */
     private $current = 0;
     
     /**
@@ -35,7 +31,7 @@ class BufferIterator implements \SeekableIterator
      *
      * @return bool
      */
-    public function valid() {
+    public function valid(): bool {
         return isset($this->buffer[$this->current]);
     }
     
@@ -44,7 +40,7 @@ class BufferIterator implements \SeekableIterator
      *
      * @return int
      */
-    public function key() {
+    public function key(): int {
         return $this->current;
     }
     
@@ -53,7 +49,7 @@ class BufferIterator implements \SeekableIterator
      *
      * @return string
      */
-    public function current() {
+    public function current(): string {
         return $this->buffer[$this->current];
     }
     
@@ -93,7 +89,7 @@ class BufferIterator implements \SeekableIterator
      *
      * @throws \OutOfBoundsException If the iterator is invalid.
      */
-    public function insert($data) {
+    public function insert(string $data) {
         if (!$this->valid()) {
             throw new \OutOfBoundsException('The iterator is not valid!');
         }
@@ -110,7 +106,7 @@ class BufferIterator implements \SeekableIterator
      *
      * @throws \OutOfBoundsException If the iterator is invalid.
      */
-    public function replace($data) {
+    public function replace(string $data): string {
         if (!$this->valid()) {
             throw new \OutOfBoundsException('The iterator is not valid!');
         }
@@ -129,7 +125,7 @@ class BufferIterator implements \SeekableIterator
      *
      * @throws \OutOfBoundsException If the iterator is invalid.
      */
-    public function remove() {
+    public function remove(): string {
         if (!$this->valid()) {
             throw new \OutOfBoundsException('The iterator is not valid!');
         }
diff --git a/lib/ClosedException.php b/lib/ClosedException.php
index 71acbf4..62ac520 100644
--- a/lib/ClosedException.php
+++ b/lib/ClosedException.php
@@ -2,4 +2,4 @@
 
 namespace Amp\Stream;
 
-class ClosedException extends \RuntimeException {}
+class ClosedException extends \Exception {}
diff --git a/lib/MemoryStream.php b/lib/MemoryStream.php
index 405ad1e..bd6ffc1 100644
--- a/lib/MemoryStream.php
+++ b/lib/MemoryStream.php
@@ -2,9 +2,8 @@
 
 namespace Amp\Stream;
 
-use Amp\Deferred;
-use Amp\Failure;
-use Amp\Success;
+use Amp\{ Deferred, Failure, Success };
+use Interop\Async\Awaitable;
 
 /**
  * Serves as buffer that implements the stream interface, allowing consumers to be notified when data is available in
@@ -27,7 +26,7 @@ class MemoryStream implements Stream {
     /**
      * @param string $data
      */
-    public function __construct($data = '') {
+    public function __construct(string $data = '') {
         $this->buffer = new Buffer($data);
         $this->reads = new \SplQueue;
     }
@@ -35,14 +34,14 @@ class MemoryStream implements Stream {
     /**
      * {@inheritdoc}
      */
-    public function isReadable() {
+    public function isReadable(): bool {
         return $this->readable;
     }
     
     /**
      * {@inheritdoc}
      */
-    public function isWritable() {
+    public function isWritable(): bool {
         return $this->writable;
     }
     
@@ -66,12 +65,9 @@ class MemoryStream implements Stream {
     /**
      * {@inheritdoc}
      */
-    public function read($bytes = null, $delimiter = null) {
-        if ($bytes !== null) {
-            $bytes = (int) $bytes;
-            if ($bytes <= 0) {
-                throw new \InvalidArgumentException("The number of bytes to read should be a positive integer or null");
-            }
+    public function read(int $bytes = null, string $delimiter = null): Awaitable {
+        if ($bytes !== null && $bytes <= 0) {
+            throw new \InvalidArgumentException("The number of bytes to read should be a positive integer or null");
         }
     
         if (!$this->readable) {
@@ -119,19 +115,23 @@ class MemoryStream implements Stream {
             $this->reads->unshift([$bytes, $delimiter, $deferred]);
             return;
         }
+        
+        if (!$this->writable && $this->buffer->isEmpty()) {
+            $this->close();
+        }
     }
     
     /**
      * {@inheritdoc}
      */
-    public function write($data) {
+    public function write(string $data): Awaitable {
         return $this->send($data, false);
     }
     
     /**
      * {@inheritdoc}
      */
-    public function end($data = '') {
+    public function end(string $data = ''): Awaitable {
         return $this->send($data, true);
     }
     
@@ -141,7 +141,7 @@ class MemoryStream implements Stream {
      *
      * @return \Interop\Async\Awaitable
      */
-    protected function send($data, $end = false) {
+    protected function send(string $data, bool $end = false): Awaitable {
         if (!$this->writable) {
             return new Failure(new \LogicException("The stream is not writable"));
         }
diff --git a/lib/Stream.php b/lib/Stream.php
index aa9e336..0be2141 100644
--- a/lib/Stream.php
+++ b/lib/Stream.php
@@ -2,20 +2,22 @@
 
 namespace Amp\Stream;
 
+use Interop\Async\Awaitable;
+
 interface Stream {
     /**
      * Determines if the stream is readable.
      *
      * @return bool
      */
-    public function isReadable();
+    public function isReadable(): bool;
     
     /**
      * Determines if the stream is writable.
      *
      * @return bool
      */
-    public function isWritable();
+    public function isWritable(): bool;
     
     /**
      * @param int|null $bytes
@@ -23,21 +25,21 @@ interface Stream {
      *
      * @return \Interop\Async\Awaitable<string> Resolves with bytes read from the stream.
      */
-    public function read($bytes = null, $delimiter = null);
+    public function read(int $bytes = null, string $delimiter = null): Awaitable;
     
     /**
      * @param string $data
      *
      * @return \Interop\Async\Awaitable<int>
      */
-    public function write($data);
+    public function write(string $data): Awaitable;
     
     /**
      * @param string $data
      *
      * @return \Interop\Async\Awaitable<int>
      */
-    public function end($data = '');
+    public function end(string $data = ''): Awaitable;
     
     /**
      * Closes the stream and fails any pending reads or writes.
diff --git a/lib/functions.php b/lib/functions.php
index 0499cb6..10d2f8f 100644
--- a/lib/functions.php
+++ b/lib/functions.php
@@ -3,10 +3,11 @@
 namespace Amp\Stream;
 
 use Amp\Coroutine;
+use Interop\Async\Awaitable;
 
 // @codeCoverageIgnoreStart
 if (\strlen('…') !== 3) {
-    throw new \RuntimeException(
+    throw new \Error(
         'The mbstring.func_overload ini setting is enabled. It must be disable to use the stream package.'
     );
 } // @codeCoverageIgnoreEnd
@@ -18,31 +19,28 @@ if (\strlen('…') !== 3) {
  *
  * @return \Interop\Async\Awaitable
  */
-function pipe(Stream $source, Stream $destination, $bytes = null) {
+function pipe(Stream $source, Stream $destination, int $bytes = null): Awaitable {
     return new Coroutine(__doPipe($source, $destination, $bytes));
 }
 
-function __doPipe(Stream $source, Stream $destination, $bytes = null) {
+function __doPipe(Stream $source, Stream $destination, int $bytes = null): \Generator {
     if (!$destination->isWritable()) {
         throw new \LogicException("The destination is not writable");
     }
     
     if (null !== $bytes) {
-        yield Coroutine::result(
-            yield $destination->write(
-                yield $source->read($bytes)
-            )
+        return yield $destination->write(
+            yield $source->read($bytes)
         );
-        return;
     }
     
     $written = 0;
     
     do {
-        $written += (yield $destination->write(
+        $written += yield $destination->write(
             yield $source->read()
-        ));
+        );
     } while ($source->isReadable() && $destination->isWritable());
     
-    yield Coroutine::result($written);
+    return $written;
 }