implics

Implics, commonly referred to as implicits, describe a mechanism in the Scala programming language that allows the compiler to automatically supply arguments, conversions, or type-class instances based on the surrounding context. The term is widely used in Scala literature, and the plural form implics is sometimes encountered as shorthand for implicit definitions.

Implicit parameters and values form the core of the feature. A method can declare parameters as implicit, and the compiler will fill these in when the method is invoked without explicit arguments, provided an appropriate implicit value is available in scope. Implicit values can be defined locally, imported from elsewhere, or placed in companion objects of the involved types. In addition, implicit conversions—defined as implicit defs or implicit classes—permit extending existing types with new methods without modifying their definitions, enabling a form of ad hoc extension.

Resolution and scope determine which implicits are chosen. When a suitable implicit is needed, the compiler searches the current scope, imported definitions, and the companion objects of the involved types. If exactly one candidate exists, it is inserted automatically. If there are multiple viable candidates, the result is an ambiguity error; if none exist, a missing implicit error is issued. This mechanism underpins many library patterns, especially type classes, and can also be used for lightweight dependency injection and generic programming.

Scala 3 introduces a redesigned approach to implicits with the given/using syntax and extension methods, while the traditional implicit mechanism remains for compatibility but is progressively being phased toward the newer style. The implicits feature remains central to Scala’s approach to generic programming, at the cost of potential hidden dependencies and debugging challenges.