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Zip and unzip lists in Standard ML

How to create a function to zip and unzip two lists as tupled lists in Standard ML?

Example: 

unzip [[1,4],[2,5],[3,6]] -> [1,2,3] [4,5,6]

zip [1,2,3] [0,2,4] -> [[1,0],[2,2],[3,4]]
Solution

  • It is usually not a good idea to use head and tail, but instead to use pattern matching. You can encode unzip a bit more elegantly as follows:

    fun unzip l = 
  case l
    of nil => (nil, nil)
     | (a,b)::tl => 
        let val (l1, l2) = unzip tl
        in (a::l1, b::l2) end

    Also as one of the commenters above mentioned, zip and unzip typically work on pairs of lists, and lists of pairs respectively.