Actually what I was trying to find was the bit that says that the equality attribute in a type variable used as a datatype parameter should be honoured where it isn't when used as a type parameter.

> type ''a t = ''a * ''a;
type 'a t = 'a * 'a
> (1.0, 2.0): real t;
val it = (1.0, 2.0): real t
>  datatype ''a t = C of ''a;
datatype 'a t = C of ''b  <<<< This prints wrongly
>  C 1.0;
poly: : error: Type error in function application.
   Function: C : ''a -> ''a t
   Argument: 1.0 : real
   Reason: Can't unify ''a to real (Requires equality type)
Found near C 1.0
Static Errors

I'm sure I looked into this a long time ago and a check with hamlet agrees with Poly/ML so I'm confident it's right; I just haven't been able to find the reference.


On 16/09/2016 20:18, Rob Arthan wrote:


On 16 Sep 2016, at 19:19, David Matthews <> wrote:

I've checked hamlet and mlton and they both reject it so I think in this case 
Poly/ML is right and SML/NJ is wrong.  I can't point to the bit of the 
definition that says that, though.

It's in Appendix C:  "Of [the type names listed in the initial static basis] all 
except exn and real admit equality".



polyml mailing list

polyml mailing list

Reply via email to