Haskell递归扫描

Question

I am trying to recursively pass in different values looping over eachother to be put into a function. I have this so far:

randNum f = take 20 (iterate f 300)
  where f n = scanl (mod') (n*2 + 75) getInts

( getInts is simply cycle and a list of numbers)

The randNum function is then used in the following way:

randGenPoints :: [Point]

randGenPoints = pairs (randNum 1)

However I have a problem where in the randNum I get this error:

*** Expression     : iterate f 300

*** Term           : f

*** Type           : Integer -> [Integer]

*** Does not match : [Integer] -> [Integer]

All I want is to have a continuously modifying list for the mod value in my function and i'm stuck and can't quite get it working....

Any advice would be much appreciated :),

Thanks

Answer 1

You are passing f , and at the same time you're redefining f . While this is perhaps not the real error, it is quite confusing. Think about it and correct, please.

Answer 2

iterate has type

iterate :: (a -> a) -> a -> [a]

It takes a function of type a -> a , which in your case is specialized to Integer -> Integer . However, the f you define in the where clause is of type Integer -> [Integer] . That happens because scanl , rather than just reducing a list to a value with some binary function, gives back a list with all intermediate values:

*Main> :t scanl
scanl :: (a -> b -> a) -> a -> [b] -> [a]
*Main> scanl (+) 0 [1..5]
[0,1,3,6,10,15]

As far as the type errors go, you probably want a fold, such as foldl' from Data.List .

*Main> :t foldl'
foldl' :: (a -> b -> a) -> a -> [b] -> a
*Main> foldl' (+) 0 [1..5]
15

That, however, will not solve all of your problems. For one, if getInts is an infinite list as you imply, then a fold on it will not terminate.

PS: Note that, as Ingo pointed out, the f argument to randNum is not the same f defined in the where clause; it is superfluous.

Answer 3

For random numbers, you're using something like

nextPseudoRandom n =  n^2 +75 `mod` 234

right? You could maybe achieve what you want by doing

randNum = iterate (map nextPseudoRandom) (getInts)

so pointwise generated noise.