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Understanding Haskell

  1. May 6, 2013 #1
    Adds one to a list of integers.
    plusOne :: [Integer] -> [Integer]
    plusOne = map (+ 1)

    I'm having a hard time understanding how this function works. More specifically how recursion happens. Map takes a function and a list and applies that function to every element in the list. In this function, map doesn't seem to be taking a function it just has (+1) but it adds one to the whole list. How does this work?
     
  2. jcsd
  3. May 6, 2013 #2

    Borek

    User Avatar

    Staff: Mentor

    I know nothing about Haskell, but I have a feeling "+1" and "+ 1" are not the same.
     
  4. May 7, 2013 #3
    Haskell supports partial application of functions.
    http://www.haskell.org/haskellwiki/Partial_application
    The expression (+ 1) gives you a function of type Int -> Int.
    You can then take that function and apply it to a value
    e.g. (+ 1) 5
    That passes the value 5 into the function (+ 1) so the result is 6.
    map (+ 1)
    is also a partial application. You only pass one argument to map i.e. the function (+ 1). So the result of that partial application is a function that takes in a list and applies (+ 1) to every element.
     
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