Tuesday, September 16, 2014

Scala and Java 8 type inference in higher order functions sample

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] = {

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] = {

filter(l)(i => i < 10) 
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 ints = Arrays.asList(1, 5, 9, 20, 30);
List 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 - http://pchiusano.blogspot.com/2011/05/making-most-of-scalas-extremely-limited.html

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