Re: Type constraints

[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)