[m-dev.] dummy types with user-defined equality/comparison

[maclarty at cs.mu.OZ.AU]

On Mon, May 29, 2006 at 05:09:09PM +1000, Julien Fischer wrote:
> 
> Currently the compiler will abort when confronted with a dummy type that has
> user-defined equality or comparison.  Below are two proposals for fixing
> this.  Which do people prefer?
> 
> (1) Change the reference manual to say that discriminated union types
>     whose body consists of a single zero-arity constructor may not have
>     user-defined equality or comparison defined upon them.  (Plus change
>     the compiler to emit the appropriate error mesages.)
> 

I think this option is best, since if values of the type are restricted
to one zero-arity function symbol then any two values of the type must
compare as equal for all sound definitions of equality and comparison.

> (2) Change the compiler so that it does not treat discriminated union types
>     whose body consists of a single zero-arity constructor as dummy types.

I thought that was how the compiler recognised dummy types.  Are you
proposing to get rid of dummy types altogether?

Ian.
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