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Date: Thu, 29 May 1997 13:56:59 -0500
From: Ernesto Posse <mposada@impsat.net.co>
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To: Caml List <caml-list@pauillac.inria.fr>
Subject: Type constraints
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[My apologies for not including a French Version]

Hello. I have the folowing problem. I need to define some interrelated
types as follows:

type 'a node = {x: 'a; y: t1}
and t1 = A | B of t1*t1
and t2 = C | D of (string * t2) node

The interpreter prints the inferred type:

type 'a node = { x: 'a; y: t1 } constraint 'a = string * t2
type t1 = | A | B of t1 * t1
type t2 = | C | D of (string * t2) node

Now, if I add another constructor:

type 'a node = {x: 'a; y: t1}
and t1 = A | B of t1*t1
and t2 = C | D of (string * t2) node | E of bool node

I obtain this message:

Characters 98-102:
This type parameter bool should be an instance of type string * t2

A solution to this would be something like:

type ('a,'b) node = {x: 'a; y: 'b}
and t1 = A | B of t1*t1
and t2 = C | D of (string * t2, t1) node | E of (bool, t1) node

This works, but I am not sure to understand the type clash in the former
example. Why did the type synthetizer infer that constraint?

Thank you for any insights on this.

-- 
Ernesto Posse
Ingeniero de Sistemas y Computacion
(Systems and Computing Engineer)
Universidad de los Andes
Santafe de Bogota
Colombia
e-mail: mposada@impsat.net.co