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:
1 2 3 | 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:
1 2 3 | 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:
1 | filter(l, (i:Int) => i < 10) |
Or a better fix is to use currying, then the type inference works!
1 2 3 4 5 6 7 | def filter[A](list: List[A])(p: A=>Boolean):List[A] = { list.filter(p)} filter(l)(i => i < 10) //ORfilter(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:
1 2 3 | 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:
1 2 | 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
| Reference: | Scala and Java 8 type inference in higher order functions sample from our JCG partner Biju Kunjummen at the all and sundry blog. |
