Self-referential tuples?

[Timon Gehr via Digitalmars-d]

On 06/10/2015 01:04 AM, Andrei Alexandrescu wrote:
> On 6/9/15 3:58 PM, Timon Gehr wrote:
>> On 06/09/2015 05:28 PM, Andrei Alexandrescu wrote:
>>> Following the use of This in Algebraic
>>> (https://github.com/D-Programming-Language/phobos/pull/3394), we can
>>> apply the same idea to Tuple, thus allowing one to create
>>> self-referential types with ease.
>>>
>>> Consider:
>>>
>>> // A singly-linked list is payload + pointer to list
>>> alias List(T) = Tuple!(T, This*);
>>>
>>> // A binary tree is payload + two children
>>> alias Tree(T) = Tuple!(T, This*, This*);
>>> // or
>>> alias Tree(T) = Tuple!(T, "payload", This*, "left", This*, "right");
>>>
>>> // A binary tree with payload only in leaves
>>> alias Tree2(T) = Algebraic!(T, Tuple!(This*, This*));
>>>
>>> Is there interest in this? Other application ideas to motivate the
>>> addition?
>>>
>>>
>>> Andrei
>>
>> Well, the issue is with this kind of use case:
>>
>> alias List(T)=Algebraic!(Tuple!(),Tuple!(T,This*));
>
> So a list is either nothing, or a head and a tail. What is the problem
> here? -- Andrei

It's about which type 'This' refers to. Is it the Algebraic or the 
Tuple? I assume here it would be the algebraic, which is expected, but 
it can get non-obvious quickly especially when e.g. a template parameter 
is a recursive tuple and the template uses the type in an Algebraic. 
It's just a mess. E.g. what happens if you do List!(Tree!int)? 
ReplaceType would need to treat Tuples specially, and then you lose the 
other semantics even though it might sometimes be needed. 'This' is a 
cute hack, but it doesn't replace a proper template fixpoint operator.