Implicit Function Types - More Details

Initial implementation in (#1775)[].


Type              ::=  ['implicit'] FunArgTypes '=>' Type
                    |  HkTypeParamClause '=>' Type
                    |  InfixType
Expr              ::=  ['implicit'] FunParams '=>' Expr
BlockResult       ::=  ['implicit'] FunParams '=>' Block
                    |  Expr1

Implicit function types associate to the right, e.g. implicit S ⇒ implicit T ⇒ U is the same as implicit S ⇒ (implicit T ⇒ U).


Implicit function types are shorthands for class types that define apply methods with implicit parameters. Specifically, the N-ary function type implicit T1, ..., TN ⇒ R is a shorthand for the class type ImplicitFunctionN[T1 , ... , TN, R]. Such class types are defined in the Scala library for N between 1 and 22 as follows.

package scala
trait ImplicitFunctionN[-T1 , ... , -TN, +R] {
  def apply(implicit x1: T1 , ... , xN: TN): R

Anonymous implicit functions implicit (x1: T1, ..., xn: Tn) => e map implicit parameters xi of types Ti to a result given by expression e. The scope of each implicit parameter xi is e. Implicit parameters must have pairwise distinct names.

If the expected type of the anonymous implicit function is of the form scala.ImplicitFunctionN[S1, ..., Sn, R], the expected type of e is R and the type Ti of any of the parameters xi can be omitted, in which case Ti = Si is assumed. If the expected type of the anonymous implicit function is some other type, all implicit parameter types must be explicitly given, and the expected type of e is undefined. The type of the anonymous implicit function is scala.ImplicitFunctionN[S1, ...,Sn, T], where T is the widened type of e. T must be equivalent to a type which does not refer to any of the implicit parameters xi.

The anonymous implicit function is evaluated as the instance creation expression:

new scala.ImplicitFunctionN[T1, ..., Tn, T] {
  def apply(implicit x1: T1, ..., xn: Tn): T = e

In the case of a single untyped implicit parameter, implicit (x) => e can be abbreviated to implicit x => e. If an anonymous implicit function implicit (x: T) => e with a single typed parameter appears as the result expression of a block, it can be abbreviated to implicit x: T => e

A implicit parameter may also be a wildcard represented by an underscore _. In that case, a fresh name for the parameter is chosen arbitrarily.

Note: The closing paragraph of the [Anonymous Functions section](https://www functions) of the Scala 2.12 is subsumed by implicit function types and should be removed.

Anonymous implicit functions implicit (x1: T1, ..., xn: Tn) => e are automatically inserted around any expression e whose expected type is scala.ImplicitFunctionN[T1, ..., Tn, R]. This is analogous to the automatic insertion of scala.Function0 around expression in by-name argument position.

Implicit functions generalize to N > 22 in the same way that functions do, see the corresponding documentation.


See the section on Expressiveness from Simplicitly: foundations and applications of implicit function types.

Type Checking

After desugaring no additional typing rules are required for implicit function types.