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Where does Scala look for implicits?

Asked 2011-04-08T16:29:37.247
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An implicit question to newcomers to Scala seems to be: where does the compiler look for implicits? I mean implicit because the question never seems to get fully formed, as if there weren't words for it. :-) For example, where do the values for integral below come from?

scala> import scala.math._
import scala.math._

scala> def foo[T](t: T)(implicit integral: Integral[T]) {println(integral)}
foo: [T](t: T)(implicit integral: scala.math.Integral[T])Unit

scala> foo(0)
scala.math.Numeric$IntIsIntegral$@3dbea611

scala> foo(0L)
scala.math.Numeric$LongIsIntegral$@48c610af

Another question that does follow up to those who decide to learn the answer to the first question is how does the compiler choose which implicit to use, in certain situations of apparent ambiguity (but that compile anyway)?

For instance, scala.Predef defines two conversions from String: one to WrappedString and another to StringOps. Both classes, however, share a lot of methods, so why doesn't Scala complain about ambiguity when, say, calling map?

Note: this question was inspired by this other question, in the hopes of stating the problem in a more general manner. The example was copied from there, because it is referred to in the answer.

scala implicit-conversion implicits
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Types of Implicits

Implicits in Scala refers to either a value that can be passed "automatically", so to speak, or a conversion from one type to another that is made automatically.

Implicit Conversion

Speaking very briefly about the latter type, if one calls a method m on an object o of a class C, and that class does not support method m, then Scala will look for an implicit conversion from C to something that does support m. A simple example would be the method map on String:

"abc".map(_.toInt)

String does not support the method map, but StringOps does, and there's an implicit conversion from String to StringOps available (see implicit def augmentString on Predef).

Implicit Parameters

The other kind of implicit is the implicit parameter. These are passed to method calls like any other parameter, but the compiler tries to fill them in automatically. If it can't, it will complain. One can pass these parameters explicitly, which is how one uses breakOut, for example (see question about breakOut, on a day you are feeling up for a challenge).

In this case, one has to declare the need for an implicit, such as the foo method declaration:

def foo[T](t: T)(implicit integral: Integral[T]) {println(integral)}

View Bounds

There's one situation where an implicit is both an implicit conversion and an implicit parameter. For example:

def getIndex[T, CC](seq: CC, value: T)(implicit conv: CC => Seq[T]) = seq.indexOf(value)

getIndex("abc", 'a')

The method getIndex can receive any object, as long as there is an implicit conversi

answered 2011-04-08T16:31:53.147

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