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Sebastian Hanowski wrote:
>> It's sort of funny-looking, though: to write a factorial function,
>> for instance, you have to give "inf" an int, which doesn't get used
>> for anything.
>
> To see how to write the factorial with the partial monad have a look at:
>
> http://sneezy.cs.nott.ac.uk/fplunch/weblog/?p=3
We don't really want factorial to be partial...
Anyway, neat seeing that, it makes it obvious how to write what I was
thinking of that two partial things can be combined such that you could
get either one... It's MonadPlus!
instance MonadPlus D where
mzero = never
mplus (Now a) _ = Now a
mplus _ (Now a) = Now a
mplus (Later d1) (Later d2) = mplus d1 d2
Although, this can pick an arbitrary answer based on the number of
cycles of recursion for each. What I was initially thinking was just for
functions like (&&). In Haskell they're asymmetric, strict in the first
argument, where if it's False the whole function is false. What would
be nice is a combinator that says that if partial argument A is foo, OR
partial argument B is bar, the whole function's answer is quux, no
matter if the other partial argument was _|_.
e.g.
(&&): False or False --> False
(||): True or True --> True
(\a b -> a && not b):
False or True --> False
hmm... are there any instances of that pattern besides boolean logic,
(if we don't also want to enter the nondeterminism arena?)
Isaac
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