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<center><h1>Module <a href="type_Lwt.html">Lwt</a></h1></center>
<br>
<pre><span class="keyword">module</span> Lwt: <code class="code"><span class="keyword">sig</span></code> <a href="Lwt.html">..</a> <code class="code"><span class="keyword">end</span></code></pre>Module <code class="code"><span class="constructor">Lwt</span></code>: cooperative light-weight threads.<br>
<hr width="100%">
<br>
This module defines <em>cooperative light-weight threads</em> with
their primitives. A <em>light-weight thread</em> represent a
computation that may be not terminated, for example because it is
waiting for some event to happen.
<p>
Lwt threads are cooperative in the sense that switching to another
thread is awlays explicit (with <a href="Lwt.html#VALwakeup"><code class="code"><span class="constructor">Lwt</span>.wakeup</code></a> or <code class="code">wekup_exn</code>). When a
thread is running, it executes as much as possible, and then
returns (a value or an eror) or sleeps.
<p>
Note that inside a Lwt thread, exceptions must be raised with
<a href="Lwt.html#VALfail"><code class="code"><span class="constructor">Lwt</span>.fail</code></a> instead of <code class="code">raise</code>. Also the <code class="code"><span class="keyword">try</span> ... <span class="keyword">with</span> ...</code>
construction will not catch Lwt errors. You must use <a href="Lwt.html#VALcatch"><code class="code"><span class="constructor">Lwt</span>.catch</code></a>
instead.
<p>
Lwt also provides the syntax extension <a href="Pa_lwt.html"><code class="code"><span class="constructor">Pa_lwt</span></code></a> to make code
using Lwt more readable.<br>
<br>
<span id="6_Definitionsandbasics"><h6>Definitions and basics</h6></span><br>
<pre><span id="TYPEt"><span class="keyword">type</span> <code class="type">+'a</code> t</span> </pre>
<div class="info">
The type of threads returning a result of type <code class="code"><span class="keywordsign">'</span>a</code>.<br>
</div>
<pre><span id="VALreturn"><span class="keyword">val</span> return</span> : <code class="type">'a -> 'a <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">return e</code> is a thread whose return value is the value of the
expression <code class="code">e</code>.<br>
</div>
<pre><span id="VALfail"><span class="keyword">val</span> fail</span> : <code class="type">exn -> 'a <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">fail e</code> is a thread that fails with the exception <code class="code">e</code>.<br>
</div>
<pre><span id="VALbind"><span class="keyword">val</span> bind</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> -> ('a -> 'b <a href="Lwt.html#TYPEt">t</a>) -> 'b <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">bind t f</code> is a thread which first waits for the thread <code class="code">t</code> to
terminate and then, if the thread succeeds, behaves as the
application of function <code class="code">f</code> to the return value of <code class="code">t</code>. If the
thread <code class="code">t</code> fails, <code class="code">bind t f</code> also fails, with the same
exception.
<p>
The expression <code class="code">bind t (<span class="keyword">fun</span> x <span class="keywordsign">-></span> t')</code> can intuitively be read as
<code class="code"><span class="keyword">let</span> x = t <span class="keyword">in</span> t'</code>, and if you use the <em>lwt.syntax</em> syntax
extension, you can write a bind operation like that: <code class="code">lwt x = t <span class="keyword">in</span> t'</code>.
<p>
Note that <code class="code">bind</code> is also often used just for synchronization
purpose: <code class="code">t'</code> will not execute before <code class="code">t</code> is terminated.
<p>
The result of a thread can be bound several time.<br>
</div>
<pre><span id="VAL(>>=)"><span class="keyword">val</span> (>>=)</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> -> ('a -> 'b <a href="Lwt.html#TYPEt">t</a>) -> 'b <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">t >>= f</code> is an alternative notation for <code class="code">bind t f</code>.<br>
</div>
<pre><span id="VAL(=<<)"><span class="keyword">val</span> (=<<)</span> : <code class="type">('a -> 'b <a href="Lwt.html#TYPEt">t</a>) -> 'a <a href="Lwt.html#TYPEt">t</a> -> 'b <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">f =<< t</code> is <code class="code">t >>= f</code><br>
</div>
<pre><span id="VALmap"><span class="keyword">val</span> map</span> : <code class="type">('a -> 'b) -> 'a <a href="Lwt.html#TYPEt">t</a> -> 'b <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">map f m</code> map the result of a thread. This is the same as <code class="code">bind
m (<span class="keyword">fun</span> x <span class="keywordsign">-></span> return (f x))</code><br>
</div>
<pre><span id="VAL(>|=)"><span class="keyword">val</span> (>|=)</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> -> ('a -> 'b) -> 'b <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">m >|= f</code> is <code class="code">map f m</code><br>
</div>
<pre><span id="VAL(=|<)"><span class="keyword">val</span> (=|<)</span> : <code class="type">('a -> 'b) -> 'a <a href="Lwt.html#TYPEt">t</a> -> 'b <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">f =|< m</code> is <code class="code">map f m</code><br>
</div>
<br>
<span id="6_Threadstorage"><h6>Thread storage</h6></span><br>
<pre><span id="TYPEkey"><span class="keyword">type</span> <code class="type">'a</code> key</span> </pre>
<div class="info">
Type of a key. Keys are used to store local values into
threads<br>
</div>
<pre><span id="VALnew_key"><span class="keyword">val</span> new_key</span> : <code class="type">unit -> 'a <a href="Lwt.html#TYPEkey">key</a></code></pre><div class="info">
<code class="code">new_key ()</code> creates a new key.<br>
</div>
<pre><span id="VALget"><span class="keyword">val</span> get</span> : <code class="type">'a <a href="Lwt.html#TYPEkey">key</a> -> 'a option</code></pre><div class="info">
<code class="code">get key</code> returns the value associated with <code class="code">key</code> in the current
thread.<br>
</div>
<pre><span id="VALwith_value"><span class="keyword">val</span> with_value</span> : <code class="type">'a <a href="Lwt.html#TYPEkey">key</a> -> 'a option -> (unit -> 'b) -> 'b</code></pre><div class="info">
<code class="code">with_value key value f</code> executes <code class="code">f</code> with <code class="code">value</code> associated to
<code class="code">key</code>. The previous value associated to <code class="code">key</code> is restored after
<code class="code">f</code> terminates.<br>
</div>
<br>
<span id="6_Exceptionshandling"><h6>Exceptions handling</h6></span><br>
<pre><span id="VALcatch"><span class="keyword">val</span> catch</span> : <code class="type">(unit -> 'a <a href="Lwt.html#TYPEt">t</a>) -> (exn -> 'a <a href="Lwt.html#TYPEt">t</a>) -> 'a <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">catch t f</code> is a thread that behaves as the thread <code class="code">t ()</code> if
this thread succeeds. If the thread <code class="code">t ()</code> fails with some
exception, <code class="code">catch t f</code> behaves as the application of <code class="code">f</code> to this
exception.<br>
</div>
<pre><span id="VALtry_bind"><span class="keyword">val</span> try_bind</span> : <code class="type">(unit -> 'a <a href="Lwt.html#TYPEt">t</a>) -> ('a -> 'b <a href="Lwt.html#TYPEt">t</a>) -> (exn -> 'b <a href="Lwt.html#TYPEt">t</a>) -> 'b <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">try_bind t f g</code> behaves as <code class="code">bind (t ()) f</code> if <code class="code">t</code> does not
fail. Otherwise, it behaves as the application of <code class="code">g</code> to the
exception associated to <code class="code">t ()</code>.<br>
</div>
<pre><span id="VALfinalize"><span class="keyword">val</span> finalize</span> : <code class="type">(unit -> 'a <a href="Lwt.html#TYPEt">t</a>) -> (unit -> unit <a href="Lwt.html#TYPEt">t</a>) -> 'a <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">finalize f g</code> returns the same result as <code class="code">f ()</code> whether it
fails or not. In both cases, <code class="code">g ()</code> is executed after <code class="code">f</code>.<br>
</div>
<br>
<span id="6_Multithreadscomposition"><h6>Multi-threads composition</h6></span><br>
<pre><span id="VALchoose"><span class="keyword">val</span> choose</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> list -> 'a <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">choose l</code> behaves as the first thread in <code class="code">l</code> to terminate. If
several threads are already terminated, one is choosen at
random.
<p>
Note: <a href="Lwt.html#VALchoose"><code class="code"><span class="constructor">Lwt</span>.choose</code></a> leaves the local values of the current thread
unchanged.<br>
</div>
<pre><span id="VALnchoose"><span class="keyword">val</span> nchoose</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> list -> 'a list <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">nchoose l</code> returns the value of all that have succcessfully
terminated. If all threads are sleeping, it waits for at least
one to terminates. If one the threads of <code class="code">l</code> fails, <code class="code">nchoose</code>
fails with the same exception.
<p>
Note: <a href="Lwt.html#VALnchoose"><code class="code"><span class="constructor">Lwt</span>.nchoose</code></a> leaves the local values of the current thread
unchanged.<br>
</div>
<pre><span id="VALnchoose_split"><span class="keyword">val</span> nchoose_split</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> list -> ('a list * 'a <a href="Lwt.html#TYPEt">t</a> list) <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">nchoose_split l</code> does the same as <a href="Lwt.html#VALnchoose"><code class="code"><span class="constructor">Lwt</span>.nchoose</code></a> but also retrurns
the list of threads that have not yet terminated.<br>
</div>
<pre><span id="VALjoin"><span class="keyword">val</span> join</span> : <code class="type">unit <a href="Lwt.html#TYPEt">t</a> list -> unit <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">join l</code> waits for all threads in <code class="code">l</code> to terminate. If one of
the threads fails, then <code class="code">join l</code> will fails with the same
exception as the first one to terminate.
<p>
Note: <a href="Lwt.html#VALjoin"><code class="code"><span class="constructor">Lwt</span>.join</code></a> leaves the local values of the current thread
unchanged.<br>
</div>
<pre><span id="VAL(<?>)"><span class="keyword">val</span> (<?>)</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> -> 'a <a href="Lwt.html#TYPEt">t</a> -> 'a <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">t <?> t'</code> is the same as <code class="code">choose [t; t']</code><br>
</div>
<pre><span id="VAL(<&>)"><span class="keyword">val</span> (<&>)</span> : <code class="type">unit <a href="Lwt.html#TYPEt">t</a> -> unit <a href="Lwt.html#TYPEt">t</a> -> unit <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">t <&> t'</code> is the same as <code class="code">join [t; t']</code><br>
</div>
<pre><span id="VALignore_result"><span class="keyword">val</span> ignore_result</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> -> unit</code></pre><div class="info">
<code class="code">ignore_result t</code> start the thread <code class="code">t</code> and ignores its result
value if the thread terminates sucessfully. However, if the
thread <code class="code">t</code> fails, the exception is raised instead of being
ignored.
<p>
You should use this function if you want to start a thread and
don't care what its return value is, nor when it terminates (for
instance, because it is looping). Note that if the thread <code class="code">t</code>
yields and later fails, the exception will not be raised at this
point in the program.<br>
</div>
<br>
<span id="6_Sleepingandresuming"><h6>Sleeping and resuming</h6></span><br>
<pre><span id="TYPEu"><span class="keyword">type</span> <code class="type">'a</code> u</span> </pre>
<div class="info">
The type of thread wakeners.<br>
</div>
<pre><span id="VALwait"><span class="keyword">val</span> wait</span> : <code class="type">unit -> 'a <a href="Lwt.html#TYPEt">t</a> * 'a <a href="Lwt.html#TYPEu">u</a></code></pre><div class="info">
<code class="code">wait ()</code> is a pair of a thread which sleeps forever (unless it
is resumed by one of the functions <code class="code">wakeup</code>, <code class="code">wakeup_exn</code> below)
and the corresponding wakener. This thread does not block the
execution of the remainder of the program (except of course, if
another thread tries to wait for its termination).<br>
</div>
<pre><span id="VALwakeup"><span class="keyword">val</span> wakeup</span> : <code class="type">'a <a href="Lwt.html#TYPEu">u</a> -> 'a -> unit</code></pre><div class="info">
<code class="code">wakeup t e</code> makes the sleeping thread <code class="code">t</code> terminate and return
the value of the expression <code class="code">e</code>.<br>
</div>
<pre><span id="VALwakeup_exn"><span class="keyword">val</span> wakeup_exn</span> : <code class="type">'a <a href="Lwt.html#TYPEu">u</a> -> exn -> unit</code></pre><div class="info">
<code class="code">wakeup_exn t e</code> makes the sleeping thread <code class="code">t</code> fail with the
exception <code class="code">e</code>.<br>
</div>
<pre><span id="VALwakeup_later"><span class="keyword">val</span> wakeup_later</span> : <code class="type">'a <a href="Lwt.html#TYPEu">u</a> -> 'a -> unit</code></pre><div class="info">
Same as <a href="Lwt.html#VALwakeup"><code class="code"><span class="constructor">Lwt</span>.wakeup</code></a> but it is not guaranteed that the thread will
be wakeup immediately.<br>
</div>
<pre><span id="VALwakeup_later_exn"><span class="keyword">val</span> wakeup_later_exn</span> : <code class="type">'a <a href="Lwt.html#TYPEu">u</a> -> exn -> unit</code></pre><div class="info">
Same as <a href="Lwt.html#VALwakeup_exn"><code class="code"><span class="constructor">Lwt</span>.wakeup_exn</code></a> but it is not guaranteed that the thread
will be wakeup immediately.<br>
</div>
<pre><span id="VALwaiter_of_wakener"><span class="keyword">val</span> waiter_of_wakener</span> : <code class="type">'a <a href="Lwt.html#TYPEu">u</a> -> 'a <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
Returns the thread associated to a wakener.<br>
</div>
<br>
<span id="6_Threadsstate"><h6>Threads state</h6></span><br>
<br><code><span id="TYPEstate"><span class="keyword">type</span> <code class="type">'a</code> state</span> = </code><table class="typetable">
<tr>
<td align="left" valign="top" >
<code><span class="keyword">|</span></code></td>
<td align="left" valign="top" >
<code><span class="constructor">Return</span> <span class="keyword">of</span> <code class="type">'a</code></code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >The thread which has successfully terminated</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code><span class="keyword">|</span></code></td>
<td align="left" valign="top" >
<code><span class="constructor">Fail</span> <span class="keyword">of</span> <code class="type">exn</code></code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >The thread raised an exception</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr>
<tr>
<td align="left" valign="top" >
<code><span class="keyword">|</span></code></td>
<td align="left" valign="top" >
<code><span class="constructor">Sleep</span></code></td>
<td class="typefieldcomment" align="left" valign="top" ><code>(*</code></td><td class="typefieldcomment" align="left" valign="top" >The thread is sleeping</td><td class="typefieldcomment" align="left" valign="bottom" ><code>*)</code></td>
</tr></table>
<div class="info">
State of a thread<br>
</div>
<pre><span id="VALstate"><span class="keyword">val</span> state</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> -> 'a <a href="Lwt.html#TYPEstate">state</a></code></pre><div class="info">
<code class="code">state t</code> returns the state of a thread<br>
</div>
<br>
<span id="6_Cancelablethreads"><h6>Cancelable threads</h6></span><br>
<br>
Cancelable threads are the same as regular threads except that
they can be canceled.<br>
<pre><span id="EXCEPTIONCanceled"><span class="keyword">exception</span> Canceled</span></pre>
<div class="info">
Canceled threads fails with this exception<br>
</div>
<pre><span id="VALtask"><span class="keyword">val</span> task</span> : <code class="type">unit -> 'a <a href="Lwt.html#TYPEt">t</a> * 'a <a href="Lwt.html#TYPEu">u</a></code></pre><div class="info">
<code class="code">task ()</code> is the same as <code class="code">wait ()</code> except that threads created
with <code class="code">task</code> can be canceled.<br>
</div>
<pre><span id="VALon_cancel"><span class="keyword">val</span> on_cancel</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> -> (unit -> unit) -> unit</code></pre><div class="info">
<code class="code">on_cancel t f</code> executes <code class="code">f</code> when <code class="code">t</code> is canceled. This is the
same as catching <code class="code"><span class="constructor">Canceled</span></code>.<br>
</div>
<pre><span id="VALcancel"><span class="keyword">val</span> cancel</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> -> unit</code></pre><div class="info">
<code class="code">cancel t</code> cancels the threads <code class="code">t</code>. This means that the deepest
sleeping thread created with <code class="code">task</code> and connected to <code class="code">t</code> is
woken up with the exception <a href="Lwt.html#EXCEPTIONCanceled"><code class="code"><span class="constructor">Lwt</span>.<span class="constructor">Canceled</span></code></a>.
<p>
For example, in the following code:
<p>
<pre></pre><code class="code"> <span class="keyword">let</span> waiter, wakener = task () <span class="keyword">in</span><br>
cancel (waiter >> printl <span class="string">"plop"</span>)<br>
</code><pre></pre>
<p>
<code class="code">waiter</code> will be woken up with <a href="Lwt.html#EXCEPTIONCanceled"><code class="code"><span class="constructor">Lwt</span>.<span class="constructor">Canceled</span></code></a>.<br>
</div>
<pre><span id="VALpick"><span class="keyword">val</span> pick</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> list -> 'a <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">pick l</code> is the same as <a href="Lwt.html#VALchoose"><code class="code"><span class="constructor">Lwt</span>.choose</code></a>, except that it cancels all
sleeping threads when one terminates.
<p>
Note: <a href="Lwt.html#VALpick"><code class="code"><span class="constructor">Lwt</span>.pick</code></a> leaves the local values of the current thread
unchanged.<br>
</div>
<pre><span id="VALnpick"><span class="keyword">val</span> npick</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> list -> 'a list <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">npick l</code> is the same as <a href="Lwt.html#VALnchoose"><code class="code"><span class="constructor">Lwt</span>.nchoose</code></a>, except that it cancels all
sleeping threads when one terminates.
<p>
Note: <a href="Lwt.html#VALnpick"><code class="code"><span class="constructor">Lwt</span>.npick</code></a> leaves the local values of the current thread
unchanged.<br>
</div>
<pre><span id="VALprotected"><span class="keyword">val</span> protected</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> -> 'a <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">protected thread</code> creates a new cancelable thread which behave
as <code class="code">thread</code> except that cancelling it does not cancel
<code class="code">thread</code>.<br>
</div>
<br>
<span id="6_Pause"><h6>Pause</h6></span><br>
<pre><span id="VALpause"><span class="keyword">val</span> pause</span> : <code class="type">unit -> unit <a href="Lwt.html#TYPEt">t</a></code></pre><div class="info">
<code class="code">pause ()</code> is a sleeping thread which is wake up on the next
call to <a href="Lwt.html#VALwakeup_paused"><code class="code"><span class="constructor">Lwt</span>.wakeup_paused</code></a>. A thread created with <code class="code">pause</code> can be
canceled.<br>
</div>
<pre><span id="VALwakeup_paused"><span class="keyword">val</span> wakeup_paused</span> : <code class="type">unit -> unit</code></pre><div class="info">
<code class="code">wakeup_paused ()</code> wakes up all threads which suspended
themselves with <a href="Lwt.html#VALpause"><code class="code"><span class="constructor">Lwt</span>.pause</code></a>.
<p>
This function is called by the scheduler, before entering the
main loop. You usually do not have to call it directly, except
if you are writing a custom scheduler.
<p>
Note that if a paused thread resume and pause again, it will not
be wakeup at this point.<br>
</div>
<pre><span id="VALpaused_count"><span class="keyword">val</span> paused_count</span> : <code class="type">unit -> int</code></pre><div class="info">
<code class="code">paused_count ()</code> returns the number of thread currently
paused.<br>
</div>
<pre><span id="VALregister_pause_notifier"><span class="keyword">val</span> register_pause_notifier</span> : <code class="type">(int -> unit) -> unit</code></pre><div class="info">
<code class="code">register_pause_notifier f</code> register a function <code class="code">f</code> that will be
called each time pause is called. The parameter passed to <code class="code">f</code> is
the new number of threads paused. It is usefull to be able to
call <a href="Lwt.html#VALwakeup_paused"><code class="code"><span class="constructor">Lwt</span>.wakeup_paused</code></a> when there is no scheduler<br>
</div>
<br>
<span id="6_Misc"><h6>Misc</h6></span><br>
<pre><span id="VALon_success"><span class="keyword">val</span> on_success</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> -> ('a -> unit) -> unit</code></pre><div class="info">
<code class="code">on_success t f</code> executes <code class="code">f</code> when <code class="code">t</code> terminates without
failing. This is the same as:
<p>
<pre></pre><code class="code"> ignore_result (bind t (<span class="keyword">fun</span> x <span class="keywordsign">-></span> f x; return ()))<br>
</code><pre></pre>
<p>
but a bit more efficient.<br>
</div>
<pre><span id="VALon_failure"><span class="keyword">val</span> on_failure</span> : <code class="type">'a <a href="Lwt.html#TYPEt">t</a> -> (exn -> unit) -> unit</code></pre><div class="info">
<code class="code">on_failure t f</code> executes <code class="code">f</code> when <code class="code">t</code> terminates and
fails. This is the same as:
<p>
<pre></pre><code class="code"> ignore_result (catch t (<span class="keyword">fun</span> e <span class="keywordsign">-></span> f e; return ()))<br>
</code><pre></pre>
<p>
but a bit more efficient.<br>
</div>
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