Re: [Caml-list] Type variables and polymorphic variants

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2013/03/13, at 6:57, Malcolm Matalka <mmatalka@gmail.com> wrote:

> Hello!  I use polymorphic variants extensively in my code for getting
> the compiler to make sure I handle errors.  However, I'm trying to write
> the following function which is proving hard, the errors are below.
> 
> I have come across similar issues before and it is related to the type
> variable. I believe the problem is the [> in error because it can't know
> if I return a polymorphic variant in this function if that is the error
> or not.
> 
> What is the cleanest way to write this function?
> 
> val with_conn :
>  host:string ->
>  port:int ->
>  (unit -> ('a, [> error ]) Deferred.Result.t) ->
>  ('a, [> error ]) Deferred.Result.t
> 
> 
> Error: The implementation conn.ml does not match the interface conn.cmi:
>       Values do not match:
>         val with_conn :
>           host:string ->
>           port:int ->
>           (unit ->
>            ('a, [> `Bad_conn ] as 'b) Core.Std._result
>       Async_core.Deferred.t) ->
>           ('a, 'b) Core.Std._result Async_core.Deferred.t
>       is not included in
>         val with_conn :
>           host:string ->
>           port:int ->
>           (unit -> ('a, [> error ]) Async.Std.Deferred.Result.t) ->
>           ('a, [> error ]) Async.Std.Deferred.Result.t
> 

The hint is in the error message:

>           (unit ->  ('a, [> `Bad_conn ] as 'b) Core.Std._result Async_core.Deferred.t) ->
>           ('a, 'b) Core.Std._result Async_core.Deferred.t

You must use an "as" to connect the types:

val with_conn :
 host:string ->
 port:int ->
 (unit -> ('a, [> error ] as 'e) Deferred.Result.t) ->
 ('a, 'e) Deferred.Result.t

Jacques Garrigue