Mark Engelberg wrote:
Chris,
I've been reading the CTM book, and am in Chapter 9. I haven't
actually tried programming anything in Mozart yet, but your email
inspired me to pull up the Mozart environment and give the PowerSet
function a try. Thanks for the inspiration!
Anyway, your program doesn't make a whole lot of sense to me. First,
it looks like your PowerSet function would just show subsets formed by
dropping initial elements of the list (like [0,1,2],[1,2],[2]). I'm
not experienced enough to see why your program would go into a wait
state. Second, SolveAll as presented in the book expects a function
that takes no arguments. So you have to wrap your function call.
It goes into a wait state because the output of PowerSet (an unbound
variable, because
PowerSet does not calculate a return value!) is passed to SolveAll
(which expects a
one-argument function). So SolveAll waits until this variable is bound.
Here's how I coded the PowerSet function using choice: For each
element of the list, I choose whether to include the element or not.
declare
fun {Maybe N}
choice
[N]
[] nil
end
end
fun {PowerSet L}
{Flatten {Map L Maybe}}
end
in
{Browse {SolveAll fun {$}{PowerSet [1 2 3]} end}}
A very nice solution, worthy of a good Prolog programmer. I like it
because it's
simple (it translates directly a declarative definition of the powerset
concept) and
because it combines higher-order programming (the use of Map and Maybe) with
relational programming.
Peter
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