module LazyDemos where
import Control.Monad.State hiding(fix)
import Data.Array
get,inc:: State Int Int
inc = mapState f (return ())
 where f ((),s) = (s,s+1)
get = mapState f (return ())
 where f ((),s) = (s,s)
-------------------------------
-- Cyclic lists
cycles:: [Int]
cycles = 1 : 2 : 3 : cycles
namedCycles = evalState (mapM f cycles) 0
 where f x = do { n <- inc; return(x,n)} -- Cyclic trees data Tree a = Tip a &#124; Fork (Tree a) (Tree a) t2 = Fork (Fork (Tip 3) (Tip 4)) (Fork (Tip 9) t2) -- Mutually cyclic t3,t4:: Tree Int (t3,t4) = ( Fork (Fork (Tip 11) t3) t4 , Fork (Tip 21) (Fork (Tip 33) t3) ) ----------------------------------------- -- primes primes :: [Integer] primes = sieve [2..] where sieve (p:xs) = p : sieve [x &#124; x<-xs, x `mod` p /= 0] ------------------------------------- -- Lazy arrays table = array (1,5) [(1,'a'),(2,'b'),(3,'c'),(5,'e'),(4,'d')] ------------------------------------- -- Dynamic programming fib :: Integer -> Integer
fib 0 = 1
fib 1 = 1
fib n = fib (n-1) + fib (n-2)
fix f = f (fix f)
g fib 0 = 1
g fib 1 = 1
g fib n = fib (n-1) + fib (n-2)
fib1 = fix g
fib2 :: Integer -> Integer
fib2 z = f z
 where table = array (0,z) [ (i, f i) &#124; i <- range (0,z) ]
 f 0 = 1
 f 1 = 1
 f n = (table ! (n-1)) + (table ! (n-2))
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