Date: Wed, 4 Aug 1999 15:52:39 +0200
From: Francois Pottier <Francois.Pottier@inria.fr>
To: caml-list@inria.fr
Subject: Weird behavior with mutually recursive type definitions
Hello,
I am puzzled by O'Caml 2.02's bizarre behavior regarding mutually recursive
type declarations.
Let's start by defining a dummy, unary type constructor.
type 'a dummy = Dummy
(The right-hand side of this definition isn't relevant here; the problem
also shows up when 'a dummy is an abstract type constructor, provided by
a functor argument.)
Then, consider the following type definitions:
type 'a t = 'a dummy
type specialized = int t
The first type declaration simply renames ``dummy'' into ``t'', and the second
one creates a specialized instance of it. O'Caml correctly analyzes these
declarations.
Now, here's the problem. Let's change just one word, and make the type
declarations mutually recursive, even though they needn't be:
type 'a t = 'a dummy
and specialized = int t
O'Caml still accepts the code, but this time, it constrains 'a to be equal to
int, as if the type constructor t could not be used polymorphically within its
own declaration:
type 'a t = 'a dummy constraint 'a = int
^^^^^^^^^^^^^^^^^^^
type specialized = int t
Why is this? Can someone explain, or is it a typechecker bug?
Thanks,
-- François Pottier Francois.Pottier@inria.fr http://pauillac.inria.fr/~fpottier/
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