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Scala and Java 8 type Inference in Higher Order Functions Sample

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Scala and Java 8 type Inference in Higher Order Functions Sample

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One of the concepts mentioned in the Functional Programming in Scala is about the type inference in higher order functions in Scala and how it fails in certain situations and a workaround for the same. So consider a sample higher order function, purely for demonstration:

def filter[A](list: List[A], p: A => Boolean):List[A] = {
 list.filter(p)
} 

Ideally, passing in a list of say integers, you would expect the predicate function to not require an explicit type:

val l = List(1, 5, 9, 20, 30) 

filter(l, i => i < 10)

Type inference does not work in this specific instance however, the fix is to specify the type explicitly:

filter(l, (i:Int) => i < 10)

Or a better fix is to use currying, then the type inference works!

def filter[A](list: List[A])(p: A=>Boolean):List[A] = {
 list.filter(p)
}                                         

filter(l)(i => i < 10) 
//OR
filter(l)(_ < 10) 

I was curious whether Java 8 type inference has this issue and tried a similar sample with Java 8 Lambda expression, the following is an equivalent filter function -

public <A> List<A> filter(List<A> list, Predicate<A> condition) {
 return list.stream().filter(condition).collect(toList());
}

and type inference for the predicate works cleanly -

List<integer> ints = Arrays.asList(1, 5, 9, 20, 30);
List<integer> lessThan10 = filter(ints, i -> i < 10);
Another blog entry on a related topic by the author of the "Functional Programming in Scala" book is available  here

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