David,
Yes, you are right; p19 applies to type functions (abbreviations), not to type constructors introduced by datatype. My understanding was that both obeyed the same rule, but I guess it is not the case (can't find anything about it in the book ..). mlton also rejects (C 1.0), by the way.
BB.
On 9/16/16 10:25 PM, David Matthews wrote:
Bernard, That refers to a "type function" not a datatype binding. type ''a t = ''a * ''a creates a type function and indeed Poly/ML ignores the equality attribute in that case. A datatype binding creates a new "type name" and that is dealt with elsewhere.
The fact that both mlton and hamlet behave the same way as Poly/ML makes me feel confident that it is SML/NJ that is wrong. Hamlet is based very closely on the semantics so it's usually the best way of checking out these sorts of questions.
Regards, David
On 16/09/2016 20:59, Bernard Berthomieu wrote:
Hello,
Besides any printing/elaboration problem with polyml 5.6:
In polyml 5.5:
datatype ''a t = C of ''a;
datatype ''a t = C of ''a
C 1.0;
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
In sml-nj 110.77:
- datatype ''a t = C of ''a;
datatype 'a t = C of 'a
- C 1.0;
val it = C 1.0 : real t
polyml interprets the type variable ''a in the declaration of t as an equality type, while sml-nj interprets it as a regular type variable.
I like the treatment of polyML :-), but I guess it is not standard:
The "definition" (http://sml-family.org/sml97-defn.pdf) says, page 19: "... In particular, the equality attribute has no significance in a bound variable of a type function; for example ... "
So, unless I'm mistaken, sml-nj is right.
Bernard.
On 9/16/16 9:41 PM, David Matthews wrote:
Rob, 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.
Regards, David
On 16/09/2016 20:18, Rob Arthan wrote:
David,
On 16 Sep 2016, at 19:19, David Matthews <David.Matthews at prolingua.co.uk> 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".
Regards,
Rob.
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