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From: Tom Hirschowitz <Tom.Hirschowitz@inria.fr>
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Date: Thu, 7 Feb 2002 10:45:15 +0100
To: Alain Frisch <frisch@clipper.ens.fr>
Cc: Jonathan D Eddy <eddy@fas.harvard.edu>, <caml-list@inria.fr>
Subject: Re: [Caml-list] Odd Type Checking Problem
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What about these ones :

# let mAny = fun succ0 input -> succ0 in
  let x = mAny (mAny true) in
  mAny;;
- : '_a -> '_b -> '_a = <fun>

# let mAny = fun succ0 input -> succ0 in
  mAny;;
- : 'a -> 'b -> 'a = <fun>


Alain Frisch writes:
 > On Wed, 6 Feb 2002, Jonathan D Eddy wrote:
 > 
 > > (* type checks *)
 > > let mAny = fun succ0 input -> succ0 in
 > >     let ans0 = true in
 > >     let x = mAny (mAny ans0) in
 > >         x 1 2
 > >
 > > (* does not type check *)
 > > let mAny: 'a -> int -> 'a = fun succ0 input -> succ0 in
 > >     let ans0 = true in
 > >     let x = mAny (mAny ans0) in
 > >         x 1 2
 > 
 > I guess this is a problem of understanding type variable scoping rules.
 > The scope of the 'a variable above is all the phrase, including
 > the (mAny (mAny ans0)). So the type annotation makes mAny monomorphic,
 > but you want to use it with two different types.
 > 
 > It seems that explicitly introduced type variables are generalized only
 > at the (syntactic) level above their introduction; this together with
 > unclear scoping rules may be confusing ...
 > 
 > 
 > -- Alain
 > 
 > -------------------
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