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Date: Sun, 10 Dec 2000 16:57:25 +0200 (IST)
From: Ohad Rodeh <orodeh@cs.huji.ac.il>
To: caml-list@inria.fr
Subject: Re: Same label in different types, how do people solve this? 
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List,

> ... stuff removed ...
> 
> > Also, I am a bit curious why it doesn't help to type explicitely,
> > i.e. to
> > write
> > let x:point_3d={x=10.;y=20.;z=30.} ???
> > 
> 
> Because this is not the way it works. Labels are defined as are values
> or constructors, this is only afterwards that the compiler checks that
> they all belong to the same type. That is, the type cannot influence the
> choice between x(point_3d) and x(point_2d), only the order of
> definition can.
> 
> There has been already various proposals for allowing types to be taken
> into account when typing record labels. The difficulty is that as soon
> as you make it into something useful, you loose the principal type
> property, and that some theoreticians working on ocaml wouldn't like
> it.

The loss of the pricipal type theorem has also recently been discussed in
comp.lang.ml and comp.lang.functional. Using type-annotation by the
compiler prior (call this the PRIOR approach) to unification can help
typing expression that cannot be typed otherwise. For example, cases of
polymoriphic recursion. In fact, this is the way this is handled in
Haskell (this topic has also appeared in this list previously). 
Although I wouldn't go running to implement all my code in Haskell :-), I
do think that using PRIOR could be an improvement. 

Can someone provide an example of how principal typing is lost once
anotations are used in a non-trivial way? 

	Ohad.