complex typeclass/constraint question
{- I'm writing code to do some combinatorial construction via test-and-evaluate algorithms. And I want to write a very generic algorithm. The idea is that the item being constructed has a current state, which is represented by a data type of typeclass EvalState. And the algorithm will consider a set of new elements to add to the object being constructed, represented by a data type of typeclass ElemSet. Here are the class definitions: -} class ElemSet a where elemSetNumElements :: a -> Int class EvalState s where isCompleteState :: s -> Bool newElemSet :: ElemSet a => s -> a integrateElem :: ElemSet a => a -> Int -> s -> s -- given an elem set, and an index of the elem to choose , compute a score scoreElem :: ElemSet a => a -> Int -> s -> EvalScore type EvalScore = Float type RandomnessChooser = [(EvalScore,Int)] -> ErrorRand Int {- here's my generic search algorithm written using the typeclasses the problem I'm having relates to the fact that I need to call 'integrateElem' which is a function in typeclass EvalState, and one of the arguments has the constraint of being of typeclass ElemSet, but I don't know how to put that constraint in. The specific error is "Ambigous type variable 'a' in the constraint: 'ElemSet a' arising from a use of 'integrateElem'" -} search :: (EvalState s) => RandomnessChooser -> s -> ErrorRand ssearch chooser state = do if isCompleteState state then return state else do let elemSet = newElemSet state n = elemSetNumElements elemSet scores = map g [0..n-1] g i = (scoreElem elemSet i state,i) chosenElemIdx <- chooser scores let newState = integrateElem elemSet chosenElemIdx state search chooser newState
On Wed, Aug 29, 2012 at 12:28:37AM -0700, Dennis Raddle wrote:
{- I'm writing code to do some combinatorial construction via test-and-evaluate algorithms. And I want to write a very generic algorithm. The idea is that the item being constructed has a current state, which is represented by a data type of typeclass EvalState. And the algorithm will consider a set of new elements to add to the object being constructed, represented by a data type of typeclass ElemSet. Here are the class definitions:
-} class ElemSet a where elemSetNumElements :: a -> Int
class EvalState s where isCompleteState :: s -> Bool newElemSet :: ElemSet a => s -> a integrateElem :: ElemSet a => a -> Int -> s -> s -- given an elem set, and an index of the elem to choose , compute a score scoreElem :: ElemSet a => a -> Int -> s -> EvalScore
As it stands, the EvalState class is quite useless, and almost certainly not what you want. Consider the type of newElemSet: newElemSet :: ElemSet a => s -> a This means that, given some type s which is an instance of EvalState, a user calling newElemSet can ask for *any type they want* as the output as long as it is an instance of ElemSet, and newElemSet has to construct it for them. Which seems rather difficult when all newElemSet has to go on is that instances of ElemSet can be queried for their size --- not much help at all in *constructing* one. integrateElem and scoreElem have similar problems -- they have to be able to deal with arbitrary instances of ElemSet, but the only thing they have to go on is the number of elements I suspect what you really want is for each instance of EvalState to have some *particular* type associated with it to represent ElemSets. There are two ways to do this. One is using a multi-parameter type class with a functional dependency: class ElemSet a => EvalState s a | s -> a where ... newElemSet :: s -> a ... The other way is using an associated type family: class ElemSet (ESType s) => EvalState s where type ESType s :: * ... newElemSet :: s -> ESType s ... Personally I would use the latter in this case. It's also possible I've completely misunderstood what you're trying to do; but whatever it is, your current EvalState class is not going to work. -Brent
Thanks Brent, yes, you got it right, as far as my intentions. The main
thing you identified was that
I haven't thought this through :)
On Wed, Aug 29, 2012 at 3:50 AM, Brent Yorgey
It's also possible I've completely misunderstood what you're trying to do; but whatever it is, your current EvalState class is not going to work.
-Brent
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