Crypt Master writes:
Am I correct in assuming that your definition of Popoulation is now using tuple and not product types ?
Actually, tuples *are* product types. They just have some syntactic sugar defined for them in the language. For example, Main> ('x',5) :: (Char,Int) ('x',5) Main> ('x',5) :: (,) Char Int ('x',5)
If so it it better to use tuples ? In the book the craft of func programing, it shows product type examples like this:
data People = Person Name Age type Name = String type Age = Int
Later it shows polymoric definitions like this:
data Pairs a = Pr a a
You mentioned that I had applied the polymorphic type "a" to Fitness, but but in the above example of person and people they have done the exactly what I did ? Used a space to seperate elements. So I am a little confused as to why mine didnt work.
Pairs has kind * -> *. Given a type, it constructs a new type. (Pr has type a -> a -> Pairs a. For any type a, it takes two values of type a and returns a value of type Pairs a). Your code looked like: type Fitness = Integer data Population a = Population [Fitness a] The kinds involved are: Population :: * -> * Fitness :: * [] :: * -> * It's the list type constructor that's causing you problems. Specifically, what you have is equivalent to: data Population a = Population ([] (Fitness a)) It's trying to apply a to Fitness, which doesn't work because Fitness is a type, not a type constructor. What you need is a single type containing a and Fitness. You could either declare a new one and use that: data IndividualWithFitness a = IWF a Fitness data Population a = Population [IWF a] -- equivalent to: -- Population a = Population ([] (IWF a)) Or just use a generic pair type: data Population a = Population [(a,Fitness)] -- equivalent to: -- Population a = Population ([] ((,) a Fitness)) (,) has kind * -> * -> *, so it takes two type arguments ("a" and "Fitness") and constructs a new one ("(a,Fitness)").
rw (Population xs) = Population (scanl1 f xs) where f (n, a) = (+ n) `pair` id
Can I ask one question, I am not concerned with performance at this point, but what sort of overhead does a function like id have. It seems unneccesary to me ( I am not critising your solution, I am vert thankfull for your help ) in a large populations you will land up doing a fair amount of extra but simple "reductions" ( I hope thats the right word. ) just to "copy" the unkown "a". Or does it have to be a function for some reason and so you had to use "id" ?
You need id if you're using pair, because pair requires two functions.
If that turned out to be a performance bottleneck, you could factor out
pair and write f directly:
rw (Population xs) = Population (scanl1 f xs)
where
f (n,a) (m,b) = (n + m, a)
--
David Menendez