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

[Rodolphe Lepigre <rodolphe.lepigre@univ-savoie.fr>]

On 25/07/2016 09:29, 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?

Just use an explicit quantification on your definition of bind by doing
something like:

  let bind : type a b. a t -> (a -> b t) -> b t = fun x y -> ...

There are two main reasons why the right polymorphic type is not infered:
 1) value restriction,
 2) you are using polymorphic recursion.

Cheers,

Rodolphe

-- 
Rodolphe Lepigre
LAMA, Université Savoie Mont Blanc, FRANCE
http://lama.univ-smb.fr/~lepigre/

> 
> >
> >
> > 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
> >>