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From: "Ryan Tarpine" <rtarpine@hotmail.com>
To: ctchou@mipos2.intel.com, caml-list@inria.fr
Subject: Re: [Caml-list] Weird typing problem
Date: Tue, 21 May 2002 20:14:21 -0400
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>From: Ching-Tsun Chou <ctchou@mipos2.intel.com>
>To: caml-list@inria.fr
>Subject: Re: [Caml-list] Weird typing problem
>Date: Tue, 21 May 2002 16:58:32 -0700 (PDT)
>
>Why can't the compiler just substitute 'a for '_a if '_a can't be
>resolved?  What's wrong with that?  Surely O'Caml can handle
>polymorphism.
>
>- Ching Tsun
>-------------------

See http://caml.inria.fr/FAQ/FAQ_EXPERT-eng.html#polymorphisme.  In short, 
(quoted from the FAQ)

When type-checking
  let name = expr1 ...
The type of expr1 is generalized when expr1 is a function, an dentifier or a 
constant. Otherwise the identifier name is not polymorphic (type variables 
are not generalized).
...
The new rule implies that if expr1 is a function application, then the 
identifier name is monomorphic

This is done because "a secure type system for Caml must forbid the 
existence of polymorphic mutable values at run-time" and this was basically 
the easiest way to do it.  Read the FAQ for an example.

HTH,

Ryan Tarpine, rtarpine@hotmail.com
"To err is human, to compute divine.  Trust your computer but not its 
programmer."
  - Morris Kingston


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