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Implicit Function Conversion

May 10, 2016

Most Scala developers are familiar with implicit type conversion which grants the Scala compiler the ability to automatically convert any type A to B at compilation time.

case class Person(name: String, age: Int)

implicit def personToInt(p: Person): Int = p.age

def increment(i: Int) = i + 1

val john = Person("John", 23)
val johnsAgeNextYear = increment(john)

One area where this abstraction leaks is when applying a function. The following is NOT possible solely by the implicit def:

val people: Seq[Person] = Seq(Person("John", 23), Person("Sally", 38))
val agesNextyear = people.map(increment)

At first glance this seems like a short coming. Scala knows how to implicitly convert from Person => Int. val people is a Seq[Person], therefore it should be possible to map that collection to a Seq[Int].

The problem, however, isn't that Scala doesn't know how to do Person => Int. Instead it is that map is expecting a function of type signature Person => B and increment is actually Int => B.

It turns out this can be resolved by defining a second implicit conversion:

implicit def intFunc1ToPersonFunc1[R](f: Int => R): Person => R =
  (p: Person) => f(p.age)

Scala now has two implicit functions, one to Person => Int and a second to (Person => B) => (Int => B). It's now possible to apply increment when mapping a Seq[Person] because Scala will implicit convert increment to a function that takes a Person and returns their age plus one.

val agesNextyear = people.map(increment)

There's a bit of a problem with the def intFunc1ToPersonFunc1 however: it repeats the same logic of conversion that is already in the implicit def personToInt. It would be better to describe that conversion once, and use it wherever needed.

implicit def intFunc1ToPersonFunc1[R](f: Int => R)(implicit convert: Person => Int): Person => R =
  (p: Person) => f(convert(p))

Now any conversion from Person => Int can be used, as long as it is in scope.

In fact, this approach could be used to generically allow any implicit function conversion for all types that have an implicit conversion defined for them.

implicit def aFuncToBFunc[A,B,R](f: A => R)(implicit convert: B => A): B => R =

SIDEBAR: (b: B) => f(convert(b)) is the same thing as f.compose(convert)

Global implicit type conversions that work on all types can sometimes cause unpredictable bugs. Be careful how this is used.