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From: Florian Weimer <fw@deneb.enyo.de>
To: Jonathan T Bryant <jtbryant@valdosta.edu>
Cc: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] Type issue
References: <2921298.1195790493072.JavaMail.jtbryant@valdosta.edu>
Date: Fri, 23 Nov 2007 09:30:50 +0100
In-Reply-To: <2921298.1195790493072.JavaMail.jtbryant@valdosta.edu> (Jonathan
	T. Bryant's message of "Thu, 22 Nov 2007 23:01:33 -0500 (EST)")
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X-Spam: no; 0.00; bool:01 bool:01 val:01 val:01 hen:98 hen:98 rec:01 rec:01 caml-list:01 cond:02 cond:02 expression:02 match:02 match:02 florian:03 

* Jonathan T. Bryant:

> # type 'a t = Cond of bool t * 'a t * 'a t | Value of 'a;;
> type 'a t = Cond of bool t * 'a t * 'a t | Value of 'a
>
> # let rec f t = match t with
>       Cond (c,t,e) -> if f c then f t else f e
>     | Value x -> x
>   ;;
> val f : bool t -> bool = <fun>

# let rec g t = match t with
      Cond (c,t,e) -> if f c then g t else g e
    | Value x -> x
  ;;
    val g : 'a t -> 'a = <fun>

The return value of f can't be generalized 'a because it's constrained
to bool in the if expression.  This problem does not arise for g.