[Haskell-cafe] Using Product Algebraic types
Stefan Holdermans
sholderm at students.cs.uu.nl
Sun Jul 4 12:44:49 EDT 2004
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
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