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- <title>THE BOOST MPL LIBRARY: Partial Metafunction Application</title>
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- <h1><a class="toc-backref" href="./more-lambda-capabilities.html#id52" name="partial-metafunction">Partial Metafunction Application</a></h1>
- <p>Consider the lambda expression <tt class="literal"><span class="pre">mpl::plus<_1,_1></span></tt>. A single
- argument is directed to both of <tt class="literal"><span class="pre">plus</span></tt>'s parameters, thereby
- adding a number to itself. Thus, a <em>binary</em> metafunction,
- <tt class="literal"><span class="pre">plus</span></tt>, is used to build a <em>unary</em> lambda expression. In other
- words, we've created a whole new computation! We're not done yet,
- though: By supplying a non-placeholder as one of the arguments, we
- can build a unary lambda expression that adds a fixed value, say
- 42, to its argument:</p>
- <pre class="literal-block">
- mpl::plus<_1, mpl::int_<42> >
- </pre>
- <!-- @ apply_test = 'enum { value = mpl::apply<', ''' >::type::value };
- BOOST_STATIC_ASSERT(value == %d);'''
- example.wrap(apply_test[0],
- ', mpl::int_<3>' + apply_test[1] % 45)
- compile() -->
- <p>The process of binding argument values to a subset of a function's
- parameters is known in the world of functional programming as
- <strong>partial function application</strong>.</p>
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