* [Caml-list] Type variables and polymorphic variants
@ 2013-03-12 21:57 Malcolm Matalka
2013-03-13 2:06 ` Jacques Garrigue
0 siblings, 1 reply; 2+ messages in thread
From: Malcolm Matalka @ 2013-03-12 21:57 UTC (permalink / raw)
To: caml-list
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
Thanks!
/Malcolm
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [Caml-list] Type variables and polymorphic variants
2013-03-12 21:57 [Caml-list] Type variables and polymorphic variants Malcolm Matalka
@ 2013-03-13 2:06 ` Jacques Garrigue
0 siblings, 0 replies; 2+ messages in thread
From: Jacques Garrigue @ 2013-03-13 2:06 UTC (permalink / raw)
To: Malcolm Matalka; +Cc: caml-list
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
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