Re: [Caml-list] Type variables won't generalize

["Ryan Tarpine" <rtarpine@hotmail.com>]

>From: Francois Pottier <francois.pottier@inria.fr>
>Reply-To: Francois.Pottier@inria.fr
>To: caml-list@inria.fr
>Subject: Re: [Caml-list] Type variables won't generalize
>Date: Thu, 4 Apr 2002 09:36:38 +0200
>
>...
>
>That is precisely what you cannot do. If different modules were allowed to
>store different types into that field, type conflicts could occur. (Imagine
>one module chooses to write `A of int, and some other module attempts to
>read `A of int -> int. An integer would be cast into a function, leading to
>a crash.) In other words, to preserve separate compilation, the compiler
>forces you to restrict that variant to a certain set by declaring its type.
>The problem does not arise in the toplevel evaluator (ocaml) because it 
>does
>not perform separate compilation.
>
>--
>François Pottier
>Francois.Pottier@inria.fr
>http://pauillac.inria.fr/~fpottier/
>-------------------

Thank you; that was the explanation I was waiting for!  My next question is 
whether or not there is some non-typesafe route around this.  Could I use 
Obj.magic to store different data types in one field, as long as I keep 
track of what type is really there (e.g., have a second field that would 
store a number representing the type) so I can cast back later?  Sorry if I 
keep finding myself longing for the "void*" C-ism!

TIA,

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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