Crypt Master, CM> I need to be able to work with a list of items whos CM> structure is onyl partially know. That is at the level CM> of this module I dont care about what rest of it is. CM> So I have this: < type Fitness = Integer < data Population a = Population [Fitness a] Well, first of all: this will not compile. You've declared Fitness to be an synonym of Integer and Integer is not a parametric data type, i.e. it has kind *. In your definition for Population, however, you apply Fitness to a type argument. This will give you a kind error. CM> Hopefully this reads Population is constructed using CM> the Population constructor and is a list who elements CM> each conists a fitness value and some other value. So, no, it does not. I guess this is what you want:
type Fitness = Integer data Population a = Population [(Fitness, a)] deriving (Show)
Now Population constructs a Population value from a list of which the elements are pairs of a Fitness value and a value of a specified type a. CM> Since I cant do poloymorphioc types with synonyms I CM> went with the data type. Well, actually, you can:
type Population' a = [(Fitness, a)]
but type synonyms have the restriction that they cannot be partially applied. Another option might be
newtype Population'' a = Population'' [(Fitness, a)]
which is only slightly different from the definition above involving data. CM> My current task is to build a roulette wheel CM> distribution of the fitness value. Basically I want to CM> build and incremental summing of the fitness value so CM> that each individual is paired with its upper range CM> like so CM> CM> Population [10 x, 20 y, 30 z] CM> New Population = [10 x, 20+10 y, 30+30 z] This can be accomplished by
rw :: Population a -> Population a rw (Population xs) = Population (scanl1 f xs) where f (n, a) = (+ n) `pair` id
where pair is the maps a pair of functions to a function on pairs:
pair :: (a -> c) -> (b -> d) -> (a, b) -> (c, d) f `pair` g = h where h (a, b) = (f a, g b)
A little test:
main :: IO () main = print $ rw (Population [(10, 2), (20, 3), (30, 5)])
This prints: "Population [(10,2),(30,3),(60,5)]". HTH, Stefan