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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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