I figured you'd had a good reason for doing it the way you did.
Konrad Hinsen wrote:
> On 24.12.2009, at 05:18, jim wrote:
>
> > I was looking at the probability monad today and think this definition
> > of m-bind might be easier to understand.
> >
> > (defmonad dist-m
> > [m-result (fn m-result-dist [v]
> > {v 1})
> > m-bind (fn m-bind-dist [mv f]
> > (reduce (partial merge-with +)
> > (for [[x p] mv
> > [y q] (f x)]
> > {y (* q p)})))
> > ])
> >
> > What do you think?
>
> I agree. In fact, I had reinvented merge-with because I wasn't aware
> of its existence.
>
> > Also, I was thinking about cond-dist-m. What if dist-m was redefined
> > to be this
> >
> > (defmonad dist-m
> > [m-result (fn m-result-dist [v]
> > {v 1})
> > m-bind (fn m-bind-dist [mv f]
> > (if (empty? mv)
> > {}
> > (reduce (partial merge-with +)
> > (for [[x p] mv
> > [y q] (f x)]
> > {y (* q p)}))))
> > m-zero {}
> > m-plus (fn m-plus-dist [& mvs]
> > (if-let [mv (first (drop-while empty? mvs))]
> > mv
> > {}))
> > ])
> >
> > I think that would roll cond-dist-m into dist-m, eliminating the need
> > for a seperate monad. Don't know if you'd thought of that and
> > discarded it or not.
>
> At first glance I doubt that this works. Did you try it?
>
> Your m-bind looks equivalent to the one of my original dist-m, with
> just an optimization for the case of an empty map. If that is true,
> then your dist-m can't include the features of cond-dist-m, which
> require a different m-bind.
>
> It is in fact essential for cond-dist-m not to eliminate invalid
> values from the distributions immediately, but to accumulate their
> weights into the map entry for some special value (nil in my
> implementation). Otherwise the probabilities come out wrong for multi-
> step computations containing more than one :when clause.
>
> Konrad.
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