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From: Reed Wilson <cedilla@gmail.com>
To: caml-list@inria.fr
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Subject: [Caml-list] Objects and polymorphic variants


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Greetings list,

I've been trying to make object methods that return polymorphic variants,
but it looks a bit stricter than I'm used to.
For example, take the following methods:

method private method_12 =3D function
| 1 -> `One
| _ -> `Two

method method_123 =3D function
| 3 -> `Three
| x -> self#method_12 x

method method_124 =3D function
| 4 -> `Four
| x -> self#method_12 x

This returns the type:
method private method_12 : int -> ([ > `Four | `One | `Three | `Two ] as 'a)
method method_123 : int -> 'a
method method_124 : int -> 'a

and complains that method_123 uses 'a, which is undefined. That's fair
enough, since it's an open type in a non-private method.
However, if I give method_123 or method_124 a closed type signature it will
give the same exact type to the other two methods.

What I really want is a signature like this:
method private method_12 : int -> [ > `One | `Two ]
method method_123 : int -> [ `One | `Two | `Three ]
method method_124 : int -> [ `One | `Two | `Four ]

If I replace method_12 with a function outside the class it works fine, but
for whatever reason method_12 really wants to be the exact same type as
method_123 and method_124.

Is there any way around this typing requirement for methods?


Thanks for any help,
Reed Wilson


PS. I'd rather avoid doing any verbose "post-processing" of the method_12
result, like this:
method_123 =3D function
| 3 -> `Three
| x -> (match self#method_12 with `One -> `One | `Two -> `Two)


--=20
=C3=A7

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<div>Greetings list,</div><div>=C2=A0</div><div>I&#39;ve been trying to mak=
e object=C2=A0methods that return polymorphic variants, but it looks a bit =
stricter than I&#39;m used to.</div><div>For example, take the following me=
thods:</div>
<div>=C2=A0</div><div>method private method_12 =3D function</div><div>| 1 -=
&gt; `One</div><div>| _ -&gt; `Two</div><div>=C2=A0</div><div>method method=
_123 =3D function</div><div>| 3 -&gt; `Three</div><div>| x -&gt; self#metho=
d_12 x</div>
<div>=C2=A0</div><div>method method_124 =3D function</div><div>| 4 -&gt; `F=
our</div><div>| x -&gt; self#method_12 x</div><div>=C2=A0</div><div>This re=
turns the type:</div><div>method private method_12 : int -&gt; ([ &gt; `Fou=
r |=C2=A0`One | `Three | `Two ] as &#39;a)</div>
<div>method method_123 : int -&gt; &#39;a</div><div>method method_124 : int=
 -&gt; &#39;a</div><div>=C2=A0</div><div>and complains that method_123 uses=
=C2=A0&#39;a, which is=C2=A0undefined. That&#39;s fair enough, since it&#39=
;s an open type in a non-private method.</div>
<div>However, if I give method_123 or method_124 a closed type signature it=
 will give the same exact=C2=A0type to the other two methods.</div><div>=C2=
=A0</div><div>What I really want is a signature like this:</div><div>method=
 private method_12 : int -&gt; [ &gt; `One | `Two ]</div>
<div>method method_123 : int -&gt; [ `One | `Two | `Three ]</div><div>metho=
d method_124 : int -&gt; [ `One | `Two | `Four ]</div><div>=C2=A0</div><div=
>If I replace method_12 with=C2=A0a function outside the class it works fin=
e, but for whatever reason method_12 really wants to be the exact same type=
 as method_123 and method_124.</div>
<div>=C2=A0</div><div>Is there any way around this typing requirement for m=
ethods?</div><div>=C2=A0</div><div>=C2=A0</div><div>Thanks for any help,</d=
iv><div>Reed Wilson</div><div>=C2=A0</div><div>=C2=A0</div><div>PS. I&#39;d=
 rather avoid doing any verbose=C2=A0&quot;post-processing&quot; of the met=
hod_12 result, like this:</div>
<div>method_123 =3D function</div><div>| 3 -&gt; `Three<br>| x -&gt; (match=
 self#method_12 with `One -&gt; `One | `Two -&gt; `Two)</div><div>=C2=A0</d=
iv><div><br>-- <br>=C3=A7
</div>

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