What is a type error?

[Zoltan Somogyi]

Andreas Rossberg <rossberg at ps.uni-sb.de> writes:
>Uh, aliasing all over the place! Actually, I think that logic 
>programming, usually based on deep unification, brings by far the worst 
>incarnation of aliasing issues to the table.

I agree that deep unification, as implemented in Prolog, brings with it
lots of aliasing problems. However, these problems have been recognized
from the seventies, and people have tried to solve them. There have
been a huge variety of mode systems added to various dialects of Prolog
over the years, which each attempt to tackle (various parts of) the aliasing
problem, none of them fully successfully in my view, since their designers
usually actually *wanted* to retain at least *some* of the expressive power
of the logic variable.

Some non-Prolog logic languages have departed from this approach, the earliest
being the Relational Language. To tie this message to another thread, the
Relational Language had a strict mode system that is as identical as possible
to the notion of typestate in NIL and Hermes given the limitations of
comparing declarative apples with imperative oranges, but significantly
earlier, 1979 vs 1986 IIRC.

Marshall wrote:
>I have explored the OO path to its bitter end and am
>convinced it is not the way. So what is left? Uniqueness
>types and logic programming, I suppose. I enjoy logic
>programming but it doesn't seem quite right. But notice:
>no pointers there!  And it doesn't seem to suffer from the
>lack.

Of course, the main logic programming language today that disallows the use
of unification for arbitrary aliasing is Mercury. It enforces this through
a strong mode system. Our motivation for the design of this mode system
was precisely to eliminate the problems Andreas identified above. However
it has also turned out to be an excellent foundation for Mercury's notion of
unique modes, its equivalent of uniqueness types, which Mercury uses to
express I/O.

Zoltan Somogyi <zs at cs.mu.OZ.AU> http://www.cs.mu.oz.au/~zs/
Department of Computer Science and Software Engineering, Univ. of Melbourne