SML: Boolean function

Question

While learning the SML, I came up with the following question. Look at this example:

fun check (elem,myList) =
let
  fun check1 elem [] = fn (x,_,_) => elem > x
  fun check2 elem [] = fn (_,y,_) => elem < y
in
  if check1 andalso check2 then 1 else 0
end

The probem is that IF THEN rule does not work here. Why? As far as I understand check1 and check2 should return BOOL value, which allows using IF THEN. I tried to declare local variables, but this didn´t help to solve the problem.

Answer 1

check1 and check2 are functions, not bool values. Presumably, you'll want to apply them to something?