Re: prelude extension proposal

From: Wolfgang Lux <wlux_at_uni-muenster.de>
Date: Wed, 26 Oct 2005 13:11:53 +0200

Michael Hanus wrote:

> You are right that a suspended search goal cannot be resumed
> in the proposed framework. However, I think that the Suspend
> case is useful for the programmer to distinguish finite failure
> and suspension of a computation. For instance, if all branches
> in the search tree are finite and end with a fail, you are
> sure that your initial goal or expression is not provable
> or has no value. Thus, under a closed world assumption,
> you might assume the negation (and this is sometimes used
> to implement negation-as-failure). However, if you consider
> suspension as a failure, this reasoning is no longer true.

Good point. But then I would say that the implementation of
allValuesOf is, errr, broken. Most users probably will implement
negation-as-failure via
   naf x = allValuesOf x >>= return . null
which falls short in the light of this reasoning. In fact,
returning the list [0] for
   allValuesOf (0 ? let X free in ensureNotFree X)
looks somewhat dubious to me.

So I modify my complaints about Bernd's proposal such that
allValuesD and allValuesB should treat the Suspend case
differently. There seem to be two choices how to do that:
1) Report an error when Suspend is encountered.
    This can be achieved in at least three different ways:
    - omit the Suspend case from the definition,
    - use an explicit call to prelude.error, or
    - use let X free in ensureNotFree X or an equivalent
      expression to make the evaluation of the solutions
      suspend like the search goal itself.
2) Change the result types of allValuesD and allValuesB
    from SearchTree a -> [a] into SearchTree a -> [Maybe a]
    and the implementation of allValuesD would become
      allValuesD (Value x) = [Just x]
      allValuesD Fail = []
      allValuesD Suspend = [Nothing]
      allValuesD (Or xs) = concatMap allValuesD xs
    Note that the user can easily extract the results of
    the terminated computations with Maybe.catMaybes.

Regards
Wolfgang


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Received on Wed Oct 26 2005 - 13:45:47 CEST

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