[mercury-users] undiscriminated unions

[Dave Slutzkin]

 --- Ralph Becket <rafe at cs.mu.OZ.AU> wrote:
> Mercury does not support undiscriminated union
> types.
> 
> You can obtain a similar effect in one of two ways.
> 
> The first is to use reflection (see how
> pprint__to_doc/2 identifies
> lists, arrays, tuples etc. for special treatment).
> 
> The second is to use type classes.
> 
> Out of interest, what's the problem you're trying to
> solve that could
> benefit from undiscriminated union types?

Um, I was basically using them as a kind of simple
record type.  In fact, I initially had:

:- module event.

...

:- type event --->
	none
	;
	event( 
		type :: int,
		next :: event,
		...
	).

Then I changed the way the events were stored, so a
'none' event no longer had any meaning or usefulness. 
Leaving me with an undiscriminated union.  I wanted to
still use the nice field access syntax and stuff.  At
the moment I've just reverted to the declaration with
'none'.

Presumably I should be using a typeclass - although
that seems like a little overkill in this case.  I'm
not sure what benfits it gets me.  Although I guess I
do have a well-defined set of functions that apply to
events...

(I haven't read through the pprint__to_doc code yet.)

Are there any other advantages I should be
considering?

thanks,

Dave.

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