I want to do 2 things after a "then" statement in a "if.. then.. else" statement

Question

let rec filtersList2fromList1 (List1:string list) (List2:string list) : string list =  
 let finalList = [] in  
 match List1 with  
 | s :: tl -> if List.mem s List2 = true   
 then finalList @ [s] else filtersList2fromList1 tl List2  
        | [] -> []

so that,

filtersList2fromList1 ["x";"y";"z"] ["z";"x"] would be ["x";"z"]  
filtersList2fromList1 ["x";"y";"z"] ["x"] would be ["x"]

what I would like to add is, if the "if" statement is true, not only it would execute "finalList @ [s]", but also "filtersList2fromList1 tl List2" so that it will be a recursion. Without executing "filtersList2fromList1 tl List2" when it is true,

filtersList2fromList1 ["x";"y";"z"] ["z";"x"] would only be ["x"], which is wrong.

How can I solve this problem?

Answer 1

To answer your specific question, you'd either use a semi-colon or a let...in construct. In your case, neither will do what you want however.

You should read through the documentation on the standard library, as the List module contains everything you need to do what you want:

let filterList2fromList1 list1 list2 =
  List.filter (fun x -> List.mem x list2) list1