# Re: Type constraints

From: Wolfgang Lux (lux@heidelbg.ibm.com)
Date: Fri May 30 1997 - 13:09:35 MET DST

Message-Id: <9705301009.AA49225@idse.heidelbg.ibm.com>
Subject: Re: Type constraints
<338DD17B.62C6@impsat.net.co>
Date: Fri, 30 May 97 12:09:35 +0100
From: Wolfgang Lux <lux@heidelbg.ibm.com>

> 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
>

A simple solution to get rid of the type constraint in this case, is to
reorder the type definitions as follows:

type t1 = A | B of t1*t1
type 'a node = {x: 'a; y: 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?

I'm not absolutely sure on this, but it looks just like another case of
the monomorphic recursion restriction of the Hindley-Milner type system.

>
> Thank you for any insights on this.
>

Hope this helps.

Regards
Wolfgang

```----
Wolfgang Lux                     WZH Heidelberg, IBM Germany
Phone: +49-6221-59-4546                Fax: +49-6221-59-3500
Internet: lux@heidelbg.ibm.com          Office: mazvm01(lux)
```

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