React demo

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 <p>Ur/Web makes it easy to write code whose execution should be distributed between the web server and client web browsers.  Server-side code is compiled to efficient native code, and client-side code is compiled to JavaScript.  Ur/Web programmers don't need to worry about these details, because the language and standard library provide a uniform ML-like interface for the whole process.</p>
 
 <p>Here's an example of a button that, when clicked, opens an alert dialog on the client.</p>
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+react.urp
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+<p>Most client-side JavaScript programs modify page contents imperatively, but Ur/Web is based on functional-reactive programming instead.  Programs allocate data sources and then describe the page as a pure function of those data sources.  When the sources change, the page changes automatically.</p>
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+<p>Here's an example where a button modifies a data source that affects some text on the page.  The affected portion of the page is indicated with the pseudo-HTML tag <tt>dyn</tt>, whose <tt>signal</tt> attribute specifies one of these pure functions over mutable sources.  A source containing data of type <tt>t</tt> has type <tt>source t</tt> and is created with the <tt>source</tt> operation within the <tt>transaction</tt> monad.  Functions over sources are represented in the monad <tt>signal</tt>.  Like in Haskell, we overload monad notations, so that the same return and bind operators can be used to write signals and transactions.  The <tt>signal</tt> function coerces a source to a signal.</p>