Algebraic Data Types in D?

[Andrei Alexandrescu via Digitalmars-d]

On 8/1/14, 7:18 AM, Timon Gehr wrote:
> It might also be possible that this is what is actually wanted:
>
> alias Foo = Algebraic!(int,Algebraic!(This[],double)[]);
>
> (I.e. This refers to the inner type.)
>
> There is always this issue as well:
> alias ListInt=Algebraic!(Tuple!(),Tuple!(int,ListInt*));
>
>
> How would you use the proposed enhancement to fix this?

I don't think there's an easy way.

> (BTW: Algebraic implements a plain sum type, so I again think the name
> is a little strange :o).)

Yah, there'd probably be a more precise name for it.

>>> There is also this kind of approach:
>>>
>>> mixin ADT!q{
>>>   List(T):
>>>   | Nil
>>>   | Cons T List!T
>>> };
>>
>> Interesting! Is there code available for that?
>> ...
>
> You can find an incomplete proof-of-concept implementation here:
> http://dpaste.dzfl.pl/77abae58a087
> (The 'match' is not particularly great, opEquals is missing, etc.)
> It was quite hard to get past the compiler.

On a cursory inspection, looks good! The question there is whether you 
or anyone would be interested in taking that to a proposal.

>> Well one other way to look at it is that striving to do things in a
>> library pushes introspection forward.
>> ...
>
> Based on past experiences, I think we'd see some opposition if the
> language was to be extended in a way to allow those kinds of features to
> be implemented completely in the library such that they behave and look
> optimally.
>
> In any case, having a template Fix operator would sometimes be convenient:
>
> struct Foo1(T){
>      T* self;
> }
>
> alias Foo=Fix!Foo1;
>
> /+
> instantiates to something like:
>
> struct Foo{
>      Foo* self;
> }
> +/
>
> :o)
>
> the most natural implementation of Fix does not work for obvious reasons:
>
> alias Fix(alias F)=F!(Fix!F);
>
> Making this work is not completely out of reach though, I think.
>
> The best I can currently do to "save" 'Algebraic' is the following:
> http://dpaste.dzfl.pl/65afd3a7ce52

That's interesting. Fix is worth making to work either as a built-in on 
a library artifact.


Andrei