Remove duplicates from a list in SML

functional-programming, recursion, sml, smlnj

Solution

One method would be to just define `remove` inside `isolate`.

fun isolate [] = []
  | isolate (l as x::xs) =
      let fun remove (x,[]) = []
            | remove (x,l as y::ys) = if x = y
                                      then remove(x,ys)
                                      else y::remove(x,ys)
      in
        x::isolate(remove(x,xs))
      end

Alternately, to make deduplication one function, though all this really does is use the library function `List.filter` to do the same thing that `remove` does.

fun isolate [] = []
  | isolate (x::xs) = x::isolate(List.filter (fn y => y <> x) xs)

Problem

I just started learning functional programming in SML and I want to know how I can combine the following two functions into a single function. The function isolate deletes the duplicates of a list of any type ('a) using the helper function 'removes'. ``` fun isolate [] = [] | isolate (l as x::xs) = x::isolate(remove(x,xs)) fun remove (x,[]) = [] | remove (x,l as y::ys) = if x = y then remove(x,ys) else y::remove(x,ys) ``` So, for the purpose of better understanding the constructs in the SML, how would you include the function remove within isolate? This may seem trivial, but I have thinking about it and can't figure it out. Thank you for your help!

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