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    <p>You have multiple problem in your code:</p> <ul> <li>you cannot take a template function pointer without specifying its template parameter as the template won't exist until the function get instanciated.</li> <li>second point, Ret(A,B) is a function type not a function pointer type.</li> <li>function pointer are a bit raw as abstraction goes, same can be achieved by a functor which also solve your problem as a polymorphic fnction object is a single, non template type.</li> <li>final technical point, template transform can't use proto::callable, you have to specialize boost::proto::is_callable explicitly. This is due to a langauge limitation on how the inheritance is detected.</li> </ul> <p>Perusing your pseudo code, I'll go for something like :</p> <pre><code>struct plus_func { template&lt;class Sig&gt; struct result; template&lt;class This,class A, class B&gt; struct result&lt;This(A,B)&gt; { typedef /*whatever*/ type; }; template&lt;class A, class B&gt; typename result&lt;plus_func(A const&amp;,B const&amp;)&gt;::type plus_func(A const&amp; lhs, B const&amp; rhs) { return lhs+rhs; } }; struct minus_func { template&lt;class Sig&gt; struct result; template&lt;class This,class A, class B&gt; struct result&lt;This(A,B)&gt; { typedef /*whatever*/ type; }; template&lt;class A, class B&gt; typename result&lt;minus_func(A const&amp;,B const&amp;)&gt;::type plus_func(A const&amp; lhs, B const&amp; rhs) { return lhs-rhs; } }; struct my_grammar; struct plus_rule : proto::plus&lt;my_grammar, my_grammar&gt; {}; struct minus_rule : proto::minus&lt;my_grammar, my_grammar&gt; {}; struct my_grammar : proto::or_&lt; proto::when&lt;proto::terminal&lt;proto::_&gt;, proto::_value&gt; , proto::when&lt;plus_rule, proto::external_transform &gt; , proto::when&lt;minus_rule, proto::external_transform &gt; &gt; {}; struct my_external_transforms : proto::external_transforms&lt; proto::when&lt;plus_rule, apply_func&lt;plus_func&gt;(my_grammar(proto::_left),my_grammar(proto::_right), proto::_state)&gt; , proto::when&lt;minus_rule, apply_func&lt;minus_func&gt;(my_grammar(proto::_left),my_grammar(proto::_right), proto::_state)&gt; &gt; {}; </code></pre> <p>Type returned by each PFO has to be computed or specified. Beware that A,B can be const/ref qualified and may need stripping before doing type computation.</p> <p>Sidenote : external_transform are not required at all for recursive rules. I guess point 4 (template callable) was what made it not work.</p>
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    1. POHow do I write a proto transform templated by a function?
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    1. USJoel Falcou
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    1. POHow do I write a proto transform templated by a function?
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    1. COI was hoping to find a solution that will allow the user to specify plus_func and minus_func more concisely (I don't mind complicating the other parts). I understand from your first bullet point that this is not possible - I cannot take a templated free function and turn it into a polymorphic functor. Right?
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    2. COOh, I see my confusion. Thanks for your help.
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    3. CONote that with C++11, the plus_func could be rather easy to write using auto/decltype and not require the result<> support structure. Now, the Polymorphic Function Object is a rather neat abstraction that helps encapsulate template like functor in a non template structure. In C++03 that's basically the best shot you have and Proto is made to recognize them out of the box.
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