Re: MLton-user Digest, Vol 55, Issue 1

Your signature S is too polymorphic for the function eq.  Also, you need equality type
variables in its specification.  So the following should work:

  signature S =
    sig
      eqtype 'a t
      val eq : ''a t * ''a t -> bool
    end

Note that there is no way in an SML signature to say that 'a t is an equality type for
_any_ 'a.  This property only holds for the special type constructors 'a ref (discussed in
the SML definition) and 'a array (part of the Basis definition).

	- John

On Sep 1, 2013, at 7:27 AM, [email protected] wrote:
> Date: Sun, 01 Sep 2013 13:27:15 +0100
> From: Phil Clayton <[email protected]>
> Subject: [MLton-user] A datatype that is, and is not, an equality
> 	type?
> To: [email protected]
> Message-ID: <[email protected]>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> This is more of a general SML question - apologies for hijacking the 
> MLton list.
> 
> I'm confused about whether a particular datatype can be used as an 
> equality type.  The following code was accepted by MLton, Poly/ML and 
> SML/NJ:
> 
>   signature S =
>     sig
>       eqtype 'a t
>     end
> 
>   structure S :> S =
>     struct
>       datatype 'a t = A of int
>     end
> 
> which suggests that 'a t is an equality type.  However, none of them accept
> 
>   signature S =
>     sig
>       eqtype 'a t
>       val eq : 'a t * 'b t -> bool
>     end
> 
>   structure S :> S =
>     struct
>       datatype 'a t = A of int
>       fun eq (a, b) = a = b
>     end
> 
> on the grounds that 'a t is _not_ an equality type!  (HaMLet behaves 
> similarly but does not give such a specific error message.)  I would be 
> grateful if someone could explain what is going on here.
> 
> In the end, I worked around the problem by defining
> 
>       fun eq (A a, A b) = a = b
> 
> instead.  In my actual code, the example was not so pathological: I had 
> something more like
> 
>   datatype 'a t = A of 'a ref
> 
> so the type parameter was actually used.
> 
> Phil


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