[boost] Re: Using tuple/variant for recursive algebraic datatypes?

From: David Abrahams (dave_at_[hidden])
Date: 2003-10-20 09:38:36

Brian McNamara <lorgon_at_[hidden]> writes:

> Is it possible to use tuple/variant to define recursive algebraic
> datatypes?  In Haskell I might say something like
>
>    type Operand = String   -- e.g. "+" or "<<"
>    data ExprTreeNode = ETN Operand ExprTree ExprTree 
>    data ExprTreeLeaf = ETL Int
>    type ExprTree = Either ExprTreeLeaf ExprTreeNode
>
> If I try to translate this directly into C++, I might say
>
>    typedef std::string Operand;
>    typedef tuple<Operand,ExprTree*,ExprTree*> ExprTreeNode;
>    typedef int ExprTreeLeaf;
>    typedef variant<ExprTreeLeaf,ExprTreeNode> ExprTree;
>
> except that this doesn't work, since we need to somehow forward-declare
> that ExprTree is a typename or something.
>
> Is there any way to do this?  Is there a good way to do this?

Isn't this very close to what you're doing?

     http://www.boost-consulting.com/boost/libs/variant/doc/sample.html#tree

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com