Question

)I am just starting with SML and have been trying to write a function that takes two lists, L1 and L2, and returns a list of elements that occur in both. This is what I have so far:

fun exists x nil = false | exists x (h::t) = (x = h) orelse (exists x t);

    fun listAnd L1 nil = nil
     | listAnd nil L2 = nil
     | listAnd L1 L2 = if exists(hd(L1) L2) = true then hd(L1)::(listAnd(tl(L1) L2)) else listAnd(tl(L1) L2);

I am not really sure where is the error.

Answer 1

Since exists is a function taking two arguments, your mistake is to put extra parentheses around both arguments. For example, exists(hd(L1) L2) should corrected as exists (hd L1) L2.

I have a few suggestions:

• removing redundant = true
• using [] for empty list and wildcard _ for unused values
• using pattern matching on L1 instead of hd and tl

Now this is the corrected function:

fun listAnd _ [] = []
  | listAnd [] _ = []
  | listAnd (x::xs) ys = if exists x ys 
                         then x::(listAnd xs ys) 
                         else listAnd xs ys