Question

I’ve come across 2 confusing problems in SML and was hoping someone could help me out:

The first is a function which takes an element and a list and decides whether that element exists in the list or not, here is the code I’ve attempted to write:

fun member (e,L) = foldl (fn(a,b) => if (e = b) then true else false) false L;

But I get bool * ‘a list –> bool but what I need is ”a * ”a list –> bool

As for the second, it also requires an element and a list but returns a list of elements less than the passed one. I’m not sure whether this should be done via map or foldr/foldl.
Any suggestions?

Thanks in advance 🙂

Answer 1

Regarding the first question, in fn (a, b) => ... a is the next element and b is the accumulator. Since you compare e with b, e is infered to have type bool. You should compare e with a, and never override b when it becomes true:

fun exists (e, L) =
    foldl (fn (a, b) => e = a orelse b) false L

For the second question, you can use foldr/foldl to do so. It’s similar to the first example; you start with empty list as the accumulator and prepend an element to it whenever that element is smaller than a threshold.

As a tradeoff, foldr gives you the right order but it isn’t tail-recursive. On the other hand, foldl is tail-recursive but gives resulting lists in a reverse order.