PhyrexTsai/Higher-order function.md

## Higher-order function.md

      
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              Higher-order function.md
            
          
    Higher-order function

We use lists to represent sets. Your task is to define a function, subsets, that receives a list as a set and returns the set of all subsets of the input set. For example:
subsets :: [Int] -> [[Int]]
subsets [] =  [ [] ]
>subsets [1,2,3] = [ [], [1], [2], [3],[1,2] [1,3], [2,3], [1,2,3] ] –順序無關 


## subsets.hs
subsets :: [Int] -> [[Int]]
subsets [] =  [ [] ]
subsets (x:xs) = subsets xs ++ map (x:) (subsets xs)

main =  do
        print(subsets [1,2,3,4])
	subsets :: [Int] -> [[Int]]
	subsets [] = [ [] ]
	subsets (x:xs) = subsets xs ++ map (x:) (subsets xs)

	main = do
	print(subsets [1,2,3,4])