Question

I have this function that takes one lists and checkes for duplicates, if any duplicates are found they are added to a new list like this:

let foo1 z list = list |> List.filter (fun e -> e <= z)

this gives foo1 1 [2;3;4;1;5;1;6;1] => [1;1;1]
the problem is that I don’t want to use any of the builtin functions in f#

Answer 1

You asked a number of basic F# questions on list processing already, so I recommend reading some introductions first and trying it yourself.

• Working with Functional Lists explains how F# lists work
• Introduction to F# has a tutorial on lists and pattern matching with recursion

Using built-in functions is the right way to solve the problem in practice. If you want to learn F# and understand recursion, then read the above first. Then you should be able to write something like:

let rec duplicates z = function
  // If the list is empty, return empty list of duplicates
  | [] -> []
  // If it starts with 'z' then return one duplicate and recursively process the rest
  | x::xs when x = z -> x::(duplicates x xs)
  // If it starts with something else, then skip the first element and process the rest
  | x::xs -> duplicates z xs

There are many F# introductions that explain how filter and similar functions are implemented. The F# wikibook covers this topic and you’ll find it in most of the F# books (see a list on fsharp.org) and Working lists section on http://www.tryfsharp.org covers this too.