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

[Goswin von Brederlow <goswin-v-b@web.de>]

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.

MfG
	Goswin