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RE: anonymous record types in variants
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Date: -- (:)
From: Manuel Fahndrich <maf@m...>
Subject: RE: anonymous record types in variants

I don't agree with Anton. The reason I want variants with anonymous record
arguments is to name the fields explicitly. I don't want to incur a runtime
cost of an extra indirection. The compilation of named fields vs. tuples
would be the same. That's why a construction like 

>         match x with A r -> ... r.x ... r.y ...

would not be desirable, since it requires the record r to be stored as  a
separate block from the A r value. If you restrict record access for these
anonymous records as Xavier pointed out

>         match x with A{lbl1 = x; lbl2 = y} -> ...


then you can implement them as efficiently as a variant with a tuple
argument.

-Manuel

-----Original Message-----
From: Anton Moscal [mailto:msk@post.tepkom.ru]

> It could be implemented this way.  However, if you declare the
> datatype as
>
>         type foo = A of {lbl1 : int; lbl2 : int}
>
> you would be forced to pattern-match it as follows
>
>         match x with A{lbl1 = x; lbl2 = y} -> ...
>
> but you can't get access to the record itself and use the dot notation
> on it, as in
>
>         match x with A r -> ... r.x ... r.y ...

But why? Natural semantic for anonymous types is the following: each
anonymous record or algebraic types declaration introduces new type
declaration in the current scope with some temporary type name. I.e.

        type ('a, 'b) foo = A of 'a * {l : 'b; l2: int}

is a shortcut for:

        type ('a, 'b) temp_name = {l1 : 'b; l2: int}
        and  ('a, 'b) foo = A of 'a * ('a, 'b) temp_name

Regards,
Anton Moscal