Re: [Caml-list] 'b t doesn't equal '_b t?

[Malcolm Matalka <mmatalka@gmail.com>]

Goswin von Brederlow <goswin-v-b@web.de> writes:

> On Mon, Jul 25, 2016 at 09:29:01AM +0000, Malcolm Matalka wrote:
>> Christopher Zimmermann <christopher@gmerlin.de> writes:
>> 
>> > '_b t is just not polymorphic. It just means the compiler could not yet
>> > figure out the monomorphic type of '_b. It will be bound to a
>> > monomorphic type later on.
>> 
>> How does one track down and why this is happening?  I tried adding some
>> typo annotations, but no luck.  Maybe I didn't add them in the right
>> place or I added the wrong annotations?
>> 
>> Any tips for handling this situation?
>> 
>> >
>> >
>> > On 2016-07-24 Malcolm Matalka <mmatalka@gmail.com> wrote:
>> >> I have the following error and I don't understand what the compiler is
>> >> trying to tell me.  Any hints?
>> >> 
>> >> Error: The implementation abb_fut.ml
>> >>        does not match the interface abb_fut.cmi:
>> >>        Values do not match:
>> >>          val bind : 'a t -> ('a -> '_b t) -> '_b t
>> >>        is not included in
>> >>          val bind : 'a t -> ('a -> 'b t) -> 'b t
>> >>        File "abb_fut.ml", line 118, characters 4-8: Actual declaration
>> >> 
>
> You can not annotate a function to be more polymorphic, only less.
> It's basically just a lower limit. So if you annotate a type as "'a t"
> that means the type must be some t, but 'a can still be infered to be
> not prolymorphic. This makes it hard to find cases where the infering
> gets too specific for your taste.

I managed to find it by adding some polymorphic type annotations and
then the compiler directed me to the offending line that violated that
annotation.

>
> MfG
> 	Goswin