Re: [Caml-list] When is a function polymorphic?

[Eric Cooper <ecc@cmu.edu>]

On Wed, Mar 30, 2005 at 07:51:04PM -0500, Yaron Minsky wrote:
> Consider some complicated union type with a single parameter:
> 
> type 'a foo = A of 'a | B of int | C of string * string | ... | ZZ of float
> 
> I want a function that converts an 'a foo to a unit foo.  I tried to
> write it this way:
> 
> function A _ -> A () | x -> x
>
> But this ends up having type: unit foo -> unit foo, which isn't what I
> want at all.  Any idea of how to achieve this cleanly?

If you can accept another level of constructors, this will work:

    type 'a foo = A of 'a  | NotA of bar
    and     bar = B of int | C of string | D of bool | ...

    function A _ -> A () | NotA x -> NotA x

-- 
Eric Cooper             e c c @ c m u . e d u