Home » JVM Languages » Scala » Scala and Java 8 type inference in higher order functions sample

About Biju Kunjummen

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] = {
 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 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

Do you want to know how to develop your skillset to become a Java Rockstar?

Subscribe to our newsletter to start Rocking right now!

To get you started we give you our best selling eBooks for FREE!

1. JPA Mini Book

2. JVM Troubleshooting Guide

3. JUnit Tutorial for Unit Testing

4. Java Annotations Tutorial

5. Java Interview Questions

6. Spring Interview Questions

7. Android UI Design

and many more ....

Leave a Reply

Your email address will not be published. Required fields are marked *

*


− four = 2

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>

Do you want to know how to develop your skillset and become a ...

Subscribe to our newsletter to start Rocking right now!

To get you started we give you our best selling eBooks for FREE!
Get ready to Rock!
To download the books, please verify your email address by following the instructions found on the email we just sent you.

THANK YOU!

Close