import SimplePrelude

----------------------------------------------------------------------------
-- Control structures as higher-order functions:

-- while loop
while :: (a -> Bool) -> (a -> a) -> a -> a
while p f x | p x       = while p f (f x)
            | otherwise = x 

potGT1000 = while (<1000) (*2) 1

----------------------------------------------------------------------------
-- Data structures implemented by higher-order functions:

-- Type of arrays:
type Array a = (Int -> a)

-- The empty array does not contain an element.
emptyArray :: Array a
emptyArray = \i -> error "Access to uninitalized element"

-- Get an element under an index.
getIndex :: Array a -> Int -> a
getIndex a i = a i

-- Add or set an element at some index position
putIndex :: Array a -> Int -> a -> Array a
putIndex a i v = \j -> if j==i then v else a j

-- An array with a default value:
initArray :: a -> Array a
initArray v = \i -> v

-- An array with elements at positions 1 and 2:
a12 = putIndex (putIndex (initArray "") 1 "Hello") 2 "World"

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