Question

I have some filters expressed as a list of function List(MyClass => Boolean) . Im trying to get the logical conjunction (AND) of all the members . I have a feeling, I could use a fold here .but Im not getting the syntax especially around how to pass the MyClass argument to the functions.

Answer 1

That’s pretty simple. Assuming your List(MyClass => Boolean) of functions is named predicates andMyClassyou are testing is namedmyClass`:

val result: Boolean = predicates.foldLeft(true) {
  (value, predicate) => 
    value && predicate(myClass)
}

Or in a wrapping function:

def conjunction(myClass: MyClass, predicates: List[MyClass => Boolean]) = 
  predicates.foldLeft(true) {
    (value, predicate) => 
      value && predicate(myClass)
  }

Explanation: if you list of predicates consists of three functions: foo(myClass: MyClass): Boolean, bar(myClass: MyClass): Boolean and buzz(myClass: MyClass): Boolean the code above is roughly equivalent to:

((true && foo(myClass)) && bar(myClass) && buzz(myClass)

I wrote an article a while ago about such use cases of foldLeft.