Re: [Caml-list] Wish: mutable variant types, equivalence with records

["François Bobot" <bobot@lri.fr>]

On 24/03/2012 13:45, Wojciech Meyer wrote:
> Please see [1], Alain Frisch has been working recently on implementing
> in-line records for constructor arguments.
>
> It's more implementation/design implications than people might think.
>
> [1] http://caml.inria.fr/mantis/view.php?id=5528

In the thread of this proposed feature, there is a remark that inlined 
record and normal record can become competing features. There is also 
the burden that inlined record as proposed can be modified only in 
pattern matching.

But can't we consider that, for a semantic, syntax and typing perspective:

type t =
    | A of string
    | B of ({msg: string; mutable foo:int} as t2)
    | C

is exactly the same thing than:

type t =
    | A of string
    | B of t2
    | C

and t2 = {msg: string; mutable foo:int}


The only difference is that when you create a record of type t2 the tag 
is directly the one of B in the first case and is the default tag for 
record (the first tag if I remember well) in the second case. So in the 
first case applying the constructor B is just the identity.

So you could modify the record of type t2 independently:

val f : t2 -> t2

...
  match x with
   | A s -> ...
   | B ({ msg = ""} as r) -> B (f t2)
   | C
...


The only disadvantage, I see, compared to Alain Frisch's proposition is 
that two records share with difficulty the same field name. But special 
case can be made when we know the record type thanks to the constructor 
eg B {x=...}, C {x=...}.

PS: It's in fact not the same thing for typing in regard of module 
subtyping, if t is made abstract, t2 must be made private. But that can 
be quite useful.

-- 
François