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copied!<p>There is no way to construct a "real" value of a no-constructor type (where by "real" I mean a terminating computation; Haskell has <code>undefined :: a</code>, <code>error :: String -> a</code> and the possibility of writing nonterminating programs like <code>mwahaha = mwahaha</code>, which to be simplistic I'll call "fake" values).</p> <p>One example of how this can be useful is versions 0.5 and later of the <a href="http://hackage.haskell.org/package/conduit/" rel="nofollow">conduit</a> library. The basic type in the library is <code>Pipe l i o u m r</code>; with different parameters for these types, a <code>Pipe</code> can serve either as a <strong>source</strong> (which produces output without consuming any input), a <strong>sink</strong> (consumes input without producing any output), or a <strong>conduit</strong> (consumes input and produces output). The <code>i</code> and <code>o</code> type parameters to <code>Pipe</code> are the types of its input and output, respectively.</p> <p>So one of the ways the conduit library enforces the notion that sources consume no input and sinks produce no output by using the <code>Void</code> type from <a href="http://hackage.haskell.org/package/void-0.5.7" rel="nofollow"><code>Data.Void</code></a> as the input type for sources and output type for sinks. Again, there is no terminating way to construct a value of such a type, so a program that attempts to consume output from a sink will not terminate (which as a reminder, in Haskell this can mean "raise an error" and not necessarily "loop forever").</p>

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