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<h3>Webmachine's approach to resource functions and referential transparency</h3>

<p>
Webmachine goes to great lengths to help your <a href="resources.html">resource functions</a> to be as referentially transparent as possible.  By "referentially transparent" we mean that given the same input <code> {ReqData, Context} </code> the function will return the same output <code> {Result, ReqData, Context} </code> and that side effects will be insignificant from the point of view of Webmachine's execution.
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We don't try to force you into pure referential transparency; we give you as big a hole as you want via <code>Context</code>.  As that term is application-specific, you can put database handles, server process identifiers, or anything else you like in there and we won't try to stop you.
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However, all Webmachine really cares about is the rest of the terms.  Since resource functions are generally referentially transparent at least with regard to those terms, many things are easier -- testing, <a href="debugging.html">debugging</a>, and even static analysis and reasoning about your Web application.
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<p>
Note that there is one important exception to this.  The <a href="streambody.html">streamed body feature</a> exists to allow resources to consume or produce request/response bodies a hunk at a time without ever having the whole thing in memory.  While the continuation-passing style used in the streaming API is friendly to general functional analysis, due to the necessary side-effect of reading or writing to sockets the stream bodies cannot be treated in quite the same way as other uses of the <code>ReqData</code> interface.  Luckily, it is easy to inspect a <code>ReqData</code> to see if this is the case in any individual resource or request instance.
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