Programming Scala, 2nd Edition

Chapter 5. Implicits

Implicits are a powerful, if controversial feature in Scala. They are used to reduce boilerplate, to simulate adding new methods to existing types, and to support the creation of domain-specific languages (DSLs).

Implicits are controversial because they are “nonlocal” in the source code. You import implicit values and methods into the local scope, except for those that are imported automatically through Predef. Once in scope, an implicit might be invoked by the compiler to populate a method argument or to convert a provided argument to the expected type. However, when reading the source code, it’s not obvious when an implicit value or method is being used, which can be confusing to the reader. Fortunately, you learn from experience to be aware when implicits might be invoked, and you learn the APIs that exploit them. Nevertheless, surprises await the beginner.

Understanding how implicits work is fairly straightforward. Most of this long chapter is devoted to example design problems that implicits solve.

Implicit Arguments

In “A Taste of Futures”, we saw one use of the implicit keyword, to label method arguments that the user does not have to provide explicitly. When an implicit argument is omitted, a type-compatible value will be used from the enclosing scope, if available. Otherwise, a compiler error occurs.

Suppose we have a method to compute sales tax where the rate is implicit:

def calcTax(amount: Float)(implicit rate: Float): Float = amount * rate