Re: [boost] calculator.html: leaf-node has children? (was Re: Preliminary submission: Boost.Proto DSEL Framework

From: Eric Niebler (eric_at_[hidden])
Date: 2007-04-06 16:16:45

Larry Evans wrote:
> On 04/04/2007 02:18 AM, Eric Niebler wrote:
>> As discussed here recently, Boost.Proto is an expression template (ET) 
>> framework for building domain-specific embedded languages (DSEL). The 
>>
> [snip]
>> Feedback is most welcome.
>>
> 
> proto/libs/xpressive/proto/doc/html/boost_proto/user_s_guide/calculator.html
> 
> contains:
> 
>    The expr<> template makes up the nodes in expression trees. The
>    first template parameter is the node type; in this case,
>    proto::tag::terminal. That means that _1 is a leaf-node in the
>    expression tree. The second template parameter is a list of children
>    types. Terminals will always have only one type in the type
>    list. The last parameter is the number of types in the list. It is
>    deduced from the type list, so you won't need to worry about it.
> 
> which suggests a leaf-node can have children.  I thought leaf-nodes had
> no children.  What am I missing?  OOPS. Maybe a leaf-node, as you're
> defining it can have no other proto-expressions as children but it can
> have other types of children.  Is that right?

That's right, expr<tag::terminal, ...> is special in that it is the only 
node type that is allowed to have a non-expr<> as a child. Even though 
leaf nodes in a tree are special, it simplifies much code to reuse the 
expr<> type this way, rather than to define a separate terminal<> template.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com