Recursive data-types

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On Saturday, 27 September 2014 at 11:26:33 UTC, ponce wrote:
> I'm dabbling with Scheme interpreter and ultimately I would 
> need to declare the following types.
>
> --------------
>
> struct Function
> {
>     Environment env;
>     Atom params;
>     Atom body_;
> }
>
> // An atom is either a string, a double, a symbol, a function 
> or a list of atoms
> alias Atom = Algebraic!(string, double, Symbol, Function, 
> This[]);
>
> --------------
>
> These definitions can't work since Function and Atom need each 
> other in this recursive definition.
>
> How to get out of this trap?
> Do I have to drop Algebraic and go back to manual tagged unions?

You can also use a pointer to a Function. Basically, any 
indirection will solve this problem, whether it be via class or 
pointer.

struct Function
{
     Environment env;

     //Either do this:
     Atom* params;
     Atom* body_;
}

//Or this                                       //Now a pointer
alias Atom = Algebraic!(string, double, Symbol, Function*, 
This[]);

Also, you might want to use This* instead of This[], unless you 
want an Atom to be able to contain a whole array of other atoms.