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
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