Russ Abbott wrote:
* It also strikes me as strange that apparently (and correct me if
I'm wrong) it is not possible to execute 'choice' directly. That
is, one can't write {Digit} or {Browse {Digit}} and expect a
result. But since 'choice' is part of the syntax of the language,
as a programmer, my expectation is that it should do something,
i.e., that the virtual machine that execute Oz programs would do
something when it gets to a 'choice'.
Let me try to clarify things a bit. When a thread executes a choice
statement, the program is facing a non-deterministic choice. Something
*from the outside* must decide which alternative to choose. This is why
the designers of Oz decided to encapsulate non-determinism. For this
purpose they invented the computation space.
So your nondeterministic program is run inside a computation space.
This space does not permit side effects, and thus looks like a closed
world. From outside the space, you can detect when the space is facing
a nondeterministic choice. There you can "guide" the computation by
committing one of the alternives.
This makes a search engine easy to implement in Oz. Computation spaces
are provided as first-class entities. Search consists in creating a
computation space with the logic program inside it, and try all possible
executions. For this, you wait until the computation succeeds, fails,
or makes a choice. When facing a choice, you typically clone the space,
and commit one of the alternatives per clone, and recursively explore them.
Cheers,
raph
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