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<h1>Module <a href="type_React.html">React</a></h1>
<pre><span class="keyword">module</span> React: <code class="code"><span class="keyword">sig</span></code> <a href="React.html">..</a> <code class="code"><span class="keyword">end</span></code></pre><div class="info module top">
Declarative events and signals.
<p>
React is a module for functional reactive programming (frp). It
provides support to program with time varying values : declarative
<a href="React.E.html">events</a> and <a href="React.S.html">signals</a>. React
doesn't define any primitive event or signal, this lets the client
choose the concrete timeline.
<p>
Consult the <a href="React.html#sem">semantics</a>, the <a href="React.html#basics">basics</a> and
<a href="React.html#ex">examples</a>. Open the module to use it, this defines only two
types and modules in your scope.
<p>
<em>Release 1.2.0 - Daniel Bünzli <daniel.buenzl i@erratique.ch> </em><br>
</div>
<hr width="100%">
<br>
<h1 id="1_Interface">Interface</h1><br>
<pre><span id="TYPEevent"><span class="keyword">type</span> <code class="type">'a</code> event</span> </pre>
<div class="info ">
The type for events of type <code class="code"><span class="keywordsign">'</span>a</code>.<br>
</div>
<pre><span id="TYPEsignal"><span class="keyword">type</span> <code class="type">'a</code> signal</span> </pre>
<div class="info ">
The type for signals of type <code class="code"><span class="keywordsign">'</span>a</code>.<br>
</div>
<pre><span id="TYPEstep"><span class="keyword">type</span> <code class="type"></code>step</span> </pre>
<div class="info ">
The type for update steps.<br>
</div>
<pre><span class="keyword">module</span> <a href="React.E.html">E</a>: <code class="code"><span class="keyword">sig</span></code> <a href="React.E.html">..</a> <code class="code"><span class="keyword">end</span></code></pre><div class="info">
Event combinators.
</div>
<pre><span class="keyword">module</span> <a href="React.S.html">S</a>: <code class="code"><span class="keyword">sig</span></code> <a href="React.S.html">..</a> <code class="code"><span class="keyword">end</span></code></pre><div class="info">
Signal combinators.
</div>
<pre><span class="keyword">module</span> <a href="React.Step.html">Step</a>: <code class="code"><span class="keyword">sig</span></code> <a href="React.Step.html">..</a> <code class="code"><span class="keyword">end</span></code></pre><div class="info">
Update steps.
</div>
<br>
<h1 id="sem">Semantics</h1>
<p>
The following notations are used to give precise meaning to the
combinators. It is important to note that in these semantic
descriptions the origin of time t = 0 is <em>always</em> fixed at
the time at which the combinator creates the event or the signal and
the semantics of the dependents is evaluated relative to this timeline.
<p>
We use dt to denote an infinitesimal amount of time.
<h2 id="evsem">Events</h2>
<p>
An event is a value with discrete occurrences over time.
<p>
The semantic function [] <code class="code">: <span class="keywordsign">'</span>a event <span class="keywordsign">-></span> time <span class="keywordsign">-></span> <span class="keywordsign">'</span>a option</code> gives
meaning to an event <code class="code">e</code> by mapping it to a function of time
[<code class="code">e</code>] returning <code class="code"><span class="constructor">Some</span> v</code> whenever the event occurs with value
<code class="code">v</code> and <code class="code"><span class="constructor">None</span></code> otherwise. We write [<code class="code">e</code>]<sub class="subscript">t</sub> the evaluation of
this <em>semantic</em> function at time t.
<p>
As a shortcut notation we also define []<sub class="subscript"><t</sub> <code class="code">: <span class="keywordsign">'</span>a event <span class="keywordsign">-></span> <span class="keywordsign">'</span>a option</code>
(resp. []<sub class="subscript"><=t</sub>) to denote the last occurrence, if any, of an
event before (resp. before or at) <code class="code">t</code>. More precisely :
<ul>
<li>[<code class="code">e</code>]<sub class="subscript"><t</sub> <code class="code">=</code> [<code class="code">e</code>]<sub class="subscript">t'</sub> with t' the greatest t' < t
(resp. <code class="code"><=</code>) such that
[<code class="code">e</code>]<sub class="subscript">t'</sub> <code class="code"><> <span class="constructor">None</span></code>.</li>
<li>[<code class="code">e</code>]<sub class="subscript"><t</sub> <code class="code">= <span class="constructor">None</span></code> if there is no such t'.</li>
</ul>
<p>
<h2 id="sigsem">Signals</h2>
<p>
A signal is a value that varies continuously over time. In
contrast to <a href="React.html#evsem">events</a> which occur at specific point
in time, a signal has a value at every point in time.
<p>
The semantic function [] <code class="code">: <span class="keywordsign">'</span>a signal <span class="keywordsign">-></span> time <span class="keywordsign">-></span> <span class="keywordsign">'</span>a</code> gives
meaning to a signal <code class="code">s</code> by mapping it to a function of time
[<code class="code">s</code>] that returns its value at a given time. We write [<code class="code">s</code>]<sub class="subscript">t</sub>
the evaluation of this <em>semantic</em> function at time t.
<h3 id="sigeq">Equality</h3>
<p>
Most signal combinators have an optional <code class="code">eq</code> parameter that
defaults to structural equality. <code class="code">eq</code> specifies the equality
function used to detect changes in the value of the resulting
signal. This function is needed for the efficient update of
signals and to deal correctly with signals that perform
<a href="React.html#sideeffects">side effects</a>.
<p>
Given an equality function on a type the combinators can be automatically
<a href="React.S.html#special">specialized</a> via a functor.
<p>
<h3 id="sigcont">Continuity</h3>
<p>
Ultimately signal updates depend on
<a href="React.html#primitives">primitives</a> updates. Thus a signal can
only approximate a real continuous signal. The accuracy of the
approximation depends on the variation rate of the real signal and
the primitive's update frequency.
<p>
<h1 id="basics">Basics</h1>
<p>
<h2 id="primitives">Primitive events and signals</h2>
<p>
React doesn't define primitive events and signals, they must be
created and updated by the client.
<p>
Primitive events are created with <a href="React.E.html#VALcreate"><code class="code"><span class="constructor">React</span>.<span class="constructor">E</span>.create</code></a>. This function
returns a new event and an update function that generates an
occurrence for the event at the time it is called. The following
code creates a primitive integer event <code class="code">x</code> and generates three
occurrences with value <code class="code">1</code>, <code class="code">2</code>, <code class="code">3</code>. Those occurrences are printed
on stdout by the effectful event <code class="code">pr_x</code>. <pre class="codepre"><code class="code"><span class="keyword">open</span> <span class="constructor">React</span>;;<br>
<br>
<span class="keyword">let</span> x, send_x = <span class="constructor">E</span>.create ()<br>
<span class="keyword">let</span> pr_x = <span class="constructor">E</span>.map print_int x<br>
<span class="keyword">let</span> () = <span class="constructor">List</span>.iter send_x [1; 2; 3]</code></pre>
Primitive signals are created with <a href="React.S.html#VALcreate"><code class="code"><span class="constructor">React</span>.<span class="constructor">S</span>.create</code></a>. This function
returns a new signal and an update function that sets the signal's value
at the time it is called. The following code creates an
integer signal <code class="code">x</code> initially set to <code class="code">1</code> and updates it three time with
values <code class="code">2</code>, <code class="code">2</code>, <code class="code">3</code>. The signal's values are printed on stdout by the
effectful signal <code class="code">pr_x</code>. Note that only updates that change
the signal's value are printed, hence the program prints <code class="code">123</code>, not <code class="code">1223</code>.
See the discussion on
<a href="React.html#sideeffects">side effects</a> for more details.
<p>
<pre class="codepre"><code class="code"><span class="keyword">open</span> <span class="constructor">React</span>;;<br>
<br>
<span class="keyword">let</span> x, set_x = <span class="constructor">S</span>.create 1<br>
<span class="keyword">let</span> pr_x = <span class="constructor">S</span>.map print_int x<br>
<span class="keyword">let</span> () = <span class="constructor">List</span>.iter set_x [2; 2; 3]</code></pre>
The <a href="React.html#clock">clock</a> example shows how a realtime time
flow can be defined.
<p>
<h2 id="steps">Update steps</h2>
<p>
The <a href="React.E.html#VALcreate"><code class="code"><span class="constructor">React</span>.<span class="constructor">E</span>.create</code></a> and <a href="React.S.html#VALcreate"><code class="code"><span class="constructor">React</span>.<span class="constructor">S</span>.create</code></a> functions return update functions
used to generate primitive event occurences and set the value of
primitive signals. Upon invocation as in the preceding section
these functions immediatly create and invoke an update step.
The <em>update step</em> automatically updates events and signals that
transitively depend on the updated primitive. The dependents of a
signal are updated iff the signal's value changed according to its
<a href="React.html#sigeq">equality function</a>.
<p>
The update functions have an optional <code class="code">step</code> argument. If they are
given a concrete <code class="code">step</code> value created with <a href="React.Step.html#VALcreate"><code class="code"><span class="constructor">React</span>.<span class="constructor">Step</span>.create</code></a>, then it
updates the event or signal but doesn't update its dependencies. It
will only do so whenever <code class="code">step</code> is executed with
<a href="React.Step.html#VALexecute"><code class="code"><span class="constructor">React</span>.<span class="constructor">Step</span>.execute</code></a>. This allows to make primitive event occurences and
signal changes simultaneous. See next section for an example.
<p>
<h2 id="simultaneity">Simultaneous events</h2>
<p>
<a href="React.html#steps">Update steps</a> are made under a
<a href="http://dx.doi.org/10.1016/0167-6423(92)90005-V">synchrony hypothesis</a> :
the update step takes no time, it is instantenous. Two event occurrences
are <em>simultaneous</em> if they occur in the same update step.
<p>
In the code below <code class="code">w</code>, <code class="code">x</code> and <code class="code">y</code> will always have simultaneous
occurrences. They <em>may</em> have simulatenous occurences with <code class="code">z</code>
if <code class="code">send_w</code> and <code class="code">send_z</code> are used with the same update step.
<p>
<pre class="codepre"><code class="code"><span class="keyword">let</span> w, send_w = <span class="constructor">E</span>.create ()<br>
<span class="keyword">let</span> x = <span class="constructor">E</span>.map succ w<br>
<span class="keyword">let</span> y = <span class="constructor">E</span>.map succ x<br>
<span class="keyword">let</span> z, send_z = <span class="constructor">E</span>.create ()<br>
<br>
<span class="keyword">let</span> () =<br>
<span class="keyword">let</span> () = send_w 3 <span class="comment">(* w x y occur simultaneously, z doesn't occur *)</span> <span class="keyword">in</span><br>
<span class="keyword">let</span> step = <span class="constructor">Step</span>.create () <span class="keyword">in</span><br>
send_w ~step 3;<br>
send_z ~step 4;<br>
<span class="constructor">Step</span>.execute step <span class="comment">(* w x z y occur simultaneously *)</span><br>
</code></pre>
<p>
<h2 id="update">The update step and thread safety</h2>
<p>
<a href="React.html#primitives">Primitives</a> are the only mean to drive the reactive
system and they are entirely under the control of the client. When
the client invokes a primitive's update function without the
<code class="code">step</code> argument or when it invokes <a href="React.Step.html#VALexecute"><code class="code"><span class="constructor">React</span>.<span class="constructor">Step</span>.execute</code></a> on a <code class="code">step</code>
value, React performs an update step.
<p>
To ensure correctness in the presence of threads, update steps
must be executed in a critical section. Let uset(<code class="code">p</code>) be the set
of events and signals that need to be updated whenever the
primitive <code class="code">p</code> is updated. Updating two primitives <code class="code">p</code> and <code class="code">p'</code>
concurrently is only allowed if uset(<code class="code">p</code>) and uset(<code class="code">p'</code>) are
disjoint. Otherwise the updates must be properly serialized.
<p>
Below, concurrent, updates to <code class="code">x</code> and <code class="code">y</code> must be serialized (or
performed on the same step if it makes sense semantically), but z
can be updated concurently to both <code class="code">x</code> and <code class="code">y</code>.
<p>
<pre class="codepre"><code class="code"><span class="keyword">open</span> <span class="constructor">React</span>;;<br>
<br>
<span class="keyword">let</span> x, set_x = <span class="constructor">S</span>.create 0<br>
<span class="keyword">let</span> y, send_y = <span class="constructor">E</span>.create ()<br>
<span class="keyword">let</span> z, set_z = <span class="constructor">S</span>.create 0<br>
<span class="keyword">let</span> max_xy = <span class="constructor">S</span>.l2 (<span class="keyword">fun</span> x y <span class="keywordsign">-></span> <span class="keyword">if</span> x > y <span class="keyword">then</span> x <span class="keyword">else</span> y) x (<span class="constructor">S</span>.hold 0 y)<br>
<span class="keyword">let</span> succ_z = <span class="constructor">S</span>.map succ z</code></pre>
<p>
<h2 id="sideeffects">Side effects</h2>
<p>
Effectful events and signals perform their side effect
exactly <em>once</em> in each <a href="React.html#steps">update step</a> in which there
is an update of at least one of the event or signal it depends on.
<p>
Remember that a signal updates in a step iff its
<a href="React.html#sigeq">equality function</a> determined that the signal
value changed. Signal initialization is unconditionally considered as
an update.
<p>
It is important to keep references on effectful events and
signals. Otherwise they may be reclaimed by the garbage collector.
The following program prints only a <code class="code">1</code>.
<pre class="codepre"><code class="code"><span class="keyword">let</span> x, set_x = <span class="constructor">S</span>.create 1<br>
<span class="keyword">let</span> () = ignore (<span class="constructor">S</span>.map print_int x)<br>
<span class="keyword">let</span> () = <span class="constructor">Gc</span>.full_major (); <span class="constructor">List</span>.iter set_x [2; 2; 3]</code></pre>
<h2 id="lifting">Lifting</h2>
<p>
Lifting transforms a regular function to make it act on signals.
The combinators
<a href="React.S.html#VALconst"><code class="code"><span class="constructor">React</span>.<span class="constructor">S</span>.const</code></a> and <a href="React.S.html#VALapp"><code class="code"><span class="constructor">React</span>.<span class="constructor">S</span>.app</code></a> allow to lift functions of arbitrary arity n,
but this involves the inefficient creation of n-1 intermediary
closure signals. The fixed arity <a href="React.S.html#lifting">lifting
functions</a> are more efficient. For example :
<pre class="codepre"><code class="code"><span class="keyword">let</span> f x y = x <span class="keyword">mod</span> y<br>
<span class="keyword">let</span> fl x y = <span class="constructor">S</span>.app (<span class="constructor">S</span>.app ~eq:(==) (<span class="constructor">S</span>.const f) x) y <span class="comment">(* inefficient *)</span><br>
<span class="keyword">let</span> fl' x y = <span class="constructor">S</span>.l2 f x y <span class="comment">(* efficient *)</span><br>
</code></pre>
Besides, some of <code class="code"><span class="constructor">Pervasives</span></code>'s functions and operators are
already lifted and availables in submodules of <a href="React.S.html"><code class="code"><span class="constructor">React</span>.<span class="constructor">S</span></code></a>. They can be
be opened in specific scopes. For example if you are dealing with
float signals you can open <a href="React.S.Float.html"><code class="code"><span class="constructor">React</span>.<span class="constructor">S</span>.<span class="constructor">Float</span></code></a>.
<pre class="codepre"><code class="code"><span class="keyword">open</span> <span class="constructor">React</span><br>
<span class="keyword">open</span> <span class="constructor">React</span>.<span class="constructor">S</span>.<span class="constructor">Float</span><br>
<br>
<span class="keyword">let</span> f t = sqrt t *. sin t <span class="comment">(* f is defined on float signals *)</span><br>
...<br>
<span class="keyword">open</span> <span class="constructor">Pervasives</span> <span class="comment">(* back to pervasives floats *)</span><br>
</code></pre>
If you are using OCaml 3.12 or later you can also use the <code class="code"><span class="keyword">let</span> <span class="keyword">open</span></code>
construct
<pre class="codepre"><code class="code"><span class="keyword">let</span> <span class="keyword">open</span> <span class="constructor">React</span>.<span class="constructor">S</span>.<span class="constructor">Float</span> <span class="keyword">in</span><br>
<span class="keyword">let</span> f t = sqrt t *. sin t <span class="keyword">in</span> <span class="comment">(* f is defined on float signals *)</span><br>
...<br>
</code></pre>
<p>
<h2 id="recursion">Mutual and self reference</h2>
<p>
Mutual and self reference among time varying values occurs naturally
in programs. However a mutually recursive definition of two signals
in which both need the value of the other at time t to define
their value at time t has no least fixed point. To break this
tight loop one signal must depend on the value the other had at time
t-dt where dt is an infinitesimal delay.
<p>
The fixed point combinators <a href="React.E.html#VALfix"><code class="code"><span class="constructor">React</span>.<span class="constructor">E</span>.fix</code></a> and <a href="React.S.html#VALfix"><code class="code"><span class="constructor">React</span>.<span class="constructor">S</span>.fix</code></a> allow to refer to
the value an event or signal had an infinitesimal amount of time
before. These fixed point combinators act on a function <code class="code">f</code> that takes
as argument the infinitesimally delayed event or signal that <code class="code">f</code>
itself returns.
<p>
In the example below <code class="code">history s</code> returns a signal whose value
is the history of <code class="code">s</code> as a list.
<pre class="codepre"><code class="code"><span class="keyword">let</span> history ?(eq = ( = )) s =<br>
<span class="keyword">let</span> push v = <span class="keyword">function</span><br>
<span class="keywordsign">|</span> [] <span class="keywordsign">-></span> [ v ]<br>
<span class="keywordsign">|</span> v' :: _ <span class="keyword">as</span> l <span class="keyword">when</span> eq v v' <span class="keywordsign">-></span> l<br>
<span class="keywordsign">|</span> l <span class="keywordsign">-></span> v :: l<br>
<span class="keyword">in</span><br>
<span class="keyword">let</span> define h =<br>
<span class="keyword">let</span> h' = <span class="constructor">S</span>.l2 push s h <span class="keyword">in</span><br>
h', h'<br>
<span class="keyword">in</span><br>
<span class="constructor">S</span>.fix [] define</code></pre>
When a program has infinitesimally delayed values a
<a href="React.html#primitives">primitive</a> may trigger more than one update
step. For example if a signal <code class="code">s</code> is infinitesimally delayed, then
its update in a step <code class="code">c</code> will trigger a new step <code class="code">c'</code> at the end
of the step in which the delayed signal of <code class="code">s</code> will have the value
<code class="code">s</code> had in <code class="code">c</code>. This means that the recursion occuring between a
signal (or event) and its infinitesimally delayed counterpart must
be well-founded otherwise this may trigger an infinite number
of update steps, like in the following examples.
<pre class="codepre"><code class="code"><span class="keyword">let</span> start, send_start = <span class="constructor">E</span>.create ()<br>
<span class="keyword">let</span> diverge =<br>
<span class="keyword">let</span> define e =<br>
<span class="keyword">let</span> e' = <span class="constructor">E</span>.select [e; start] <span class="keyword">in</span><br>
e', e'<br>
<span class="keyword">in</span><br>
<span class="constructor">E</span>.fix define<br>
<br>
<span class="keyword">let</span> () = send_start () <span class="comment">(* diverges *)</span><br>
<br>
<span class="keyword">let</span> diverge = <span class="comment">(* diverges *)</span><br>
<span class="keyword">let</span> define s =<br>
<span class="keyword">let</span> s' = <span class="constructor">S</span>.<span class="constructor">Int</span>.succ s <span class="keyword">in</span><br>
s', s'<br>
<span class="keyword">in</span><br>
<span class="constructor">S</span>.fix 0 define</code></pre>
For technical reasons, delayed events and signals (those given to
fixing functions) are not allowed to directly depend on each
other. Fixed point combinators will raise <code class="code"><span class="constructor">Invalid_argument</span></code> if
such dependencies are created. This limitation can be
circumvented by mapping these values with the identity.
<p>
<h2 id="strongstop">Strong stops</h2>
<p>
Strong stops should only be used on platforms where weak arrays have
a strong semantics (i.e. JavaScript). You can safely ignore that
section and the <code class="code">strong</code> argument of <a href="React.E.html#VALstop"><code class="code"><span class="constructor">React</span>.<span class="constructor">E</span>.stop</code></a> and <a href="React.S.html#VALstop"><code class="code"><span class="constructor">React</span>.<span class="constructor">S</span>.stop</code></a>
if that's not the case.
<p>
Whenever <a href="React.E.html#VALstop"><code class="code"><span class="constructor">React</span>.<span class="constructor">E</span>.stop</code></a> and <a href="React.S.html#VALstop"><code class="code"><span class="constructor">React</span>.<span class="constructor">S</span>.stop</code></a> is called with <code class="code">~strong:<span class="keyword">true</span></code> on a
reactive value <code class="code">v</code>, it is first stopped and then it walks over the
list <code class="code">prods</code> of events and signals that it depends on and
unregisters itself from these ones as a dependent (something that is
normally automatically done when <code class="code">v</code> is garbage collected since
dependents are stored in a weak array). Then for each element of
<code class="code">prod</code> that has no dependents anymore and is not a primitive it
stops them aswell and recursively.
<p>
A stop call with <code class="code">~strong:<span class="keyword">true</span></code> is more involved. But it allows to
prevent memory leaks when used judiciously on the leaves of the
reactive system that are no longer used.
<p>
<b>Warning.</b> It should be noted that if direct references are kept
on an intermediate event or signal of the reactive system it may
suddenly stop updating if all its dependents were strongly stopped. In
the example below, <code class="code">e1</code> will <em>never</em> occur:
<pre class="codepre"><code class="code"><span class="keyword">let</span> e, e_send = <span class="constructor">E</span>.create ()<br>
<span class="keyword">let</span> e1 = <span class="constructor">E</span>.map (<span class="keyword">fun</span> x <span class="keywordsign">-></span> x + 1) e <span class="comment">(* never occurs *)</span><br>
<span class="keyword">let</span> () =<br>
<span class="keyword">let</span> e2 = <span class="constructor">E</span>.map (<span class="keyword">fun</span> x <span class="keywordsign">-></span> x + 1) e1 <span class="keyword">in</span><br>
<span class="constructor">E</span>.stop ~strong:<span class="keyword">true</span> e2<br>
</code></pre>
This can be side stepped by making an artificial dependency to keep
the reference:
<pre class="codepre"><code class="code"><span class="keyword">let</span> e, e_send = <span class="constructor">E</span>.create ()<br>
<span class="keyword">let</span> e1 = <span class="constructor">E</span>.map (<span class="keyword">fun</span> x <span class="keywordsign">-></span> x + 1) e <span class="comment">(* may still occur *)</span><br>
<span class="keyword">let</span> e1_ref = <span class="constructor">E</span>.map (<span class="keyword">fun</span> x <span class="keywordsign">-></span> x) e1<br>
<span class="keyword">let</span> () =<br>
<span class="keyword">let</span> e2 = <span class="constructor">E</span>.map (<span class="keyword">fun</span> x <span class="keywordsign">-></span> x + 1) e1 <span class="keyword">in</span><br>
<span class="constructor">E</span>.stop ~strong:<span class="keyword">true</span> e2<br>
</code></pre>
<p>
<h1 id="ex">Examples</h1>
<p>
<h2 id="clock">Clock</h2>
<p>
The following program defines a primitive event <code class="code">seconds</code> holding
the UNIX time and occuring on every second. An effectful event
converts these occurences to local time and prints them on stdout
along with an
<a href="http://www.ecma-international.org/publications/standards/Ecma-048.htm">ANSI
escape sequence</a> to control the cursor position.
<pre class="codepre"><code class="code"><span class="keyword">let</span> pr_time t =<br>
<span class="keyword">let</span> tm = <span class="constructor">Unix</span>.localtime t <span class="keyword">in</span><br>
<span class="constructor">Printf</span>.printf <span class="string">"\x1B[8D%02d:%02d:%02d%!"</span><br>
tm.<span class="constructor">Unix</span>.tm_hour tm.<span class="constructor">Unix</span>.tm_min tm.<span class="constructor">Unix</span>.tm_sec<br>
<br>
<span class="keyword">open</span> <span class="constructor">React</span>;;<br>
<br>
<span class="keyword">let</span> seconds, run =<br>
<span class="keyword">let</span> e, send = <span class="constructor">E</span>.create () <span class="keyword">in</span><br>
<span class="keyword">let</span> run () =<br>
<span class="keyword">while</span> <span class="keyword">true</span> <span class="keyword">do</span> send (<span class="constructor">Unix</span>.gettimeofday ()); <span class="constructor">Unix</span>.sleep 1 <span class="keyword">done</span><br>
<span class="keyword">in</span><br>
e, run<br>
<br>
<span class="keyword">let</span> printer = <span class="constructor">E</span>.map pr_time seconds<br>
<br>
<span class="keyword">let</span> () = run ()</code></pre><br>
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