[mercury-users] abstract types.

[Ralph Becket]

> There's a bit of a problem in making bidirectional functions 
> like this -
> apart from anything else, the code for \/ing the bits together doesn't
> work in the backwards mode (ie:
> 	lr1item(ProdNum, Dot, Sym) = Bits :-
> 		encode_sym(Sym, SymNum),
> 		Bits = (ProdNum << 20) \/ (Dot << 12) \/ SymNum.
> )
> 
> Now, you can work around this by using the C interface as a layer
> between the layers of Mercury - implement the two modes as 
> pragma c_code
> that calls two separate mercury preds/funcs that each 
> implement a single
> mode. This seems expensive and ugly. Perhaps it's time to 
> allow separate
> implementations of different modes of a pred/func?

I agree that a facility for the latter would be useful.
However, is it necessary for your code to use the same
predicate for packing and unpacking?

Cheers,

Ralph
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