Wildcard Arguments in Types

Motivation

We would like to use the underscore syntax _ to stand for an anonymous type parameter, aligning it with its meaning in value parameter lists. So, just as f(_) is a shorthand for the lambda x => f(x), in the future C[_] will be a shorthand for the type lambda [X] =>> C[X]. This will make higher-kinded types easier to use. It will also remove the wart that, used as a type parameter, F[_] means F is a type constructor, whereas used as a type, F[_] means it is a wildcard (i.e. existential) type. In the future, F[_] will mean the same thing, no matter where it is used.

We pick ? as a replacement syntax for wildcard types, since it aligns with Java's syntax.

Migration Strategy

The migration to the new scheme is complicated, in particular since the kind projector compiler plugin still uses the reverse convention, with ? meaning parameter placeholder instead of wildcard. Fortunately, kind projector has added * as an alternative syntax for ?.

A step-by-step migration is made possible with the following measures:

1. In earlier versions of Scala 3, both _ and ? are legal names for wildcards.
2. In Scala 3.4, _ will be deprecated in favor of ? as a name for wildcards. A -rewrite option is available to rewrite one to the other.
3. At some later point in the future, the meaning of _ will change from wildcard to placeholder for type parameters.
4. Some deprecation warnings are already available under the -source future setting.

To smooth the transition for codebases that use kind-projector, we adopt the following measures under the command line option -Ykind-projector:

1. In Scala 3.0, * is available as a type parameter placeholder.
2. In Scala 3.2, * is deprecated in favor of _. A -rewrite option is available to rewrite one to the other.
3. In Scala 3.3, * is removed again, and all type parameter placeholders will be expressed with _.

These rules make it possible to cross-build between Scala 2 using the kind projector plugin and Scala 3.0 - 3.2 using the compiler option -Ykind-projector.

There is also a migration path for users that want a one-time transition to syntax with _ as a type parameter placeholder. With option -Ykind-projector:underscores Scala 3 will regard _ as a type parameter placeholder, leaving ? as the only syntax for wildcards.

To cross-compile with old Scala 2 sources, while using _ a placeholder, you must use options -Xsource:3 -P:kind-projector:underscore-placeholders together with a recent version of kind-projector (0.13 and higher) and most recent versions of Scala 2 (2.13.5 and higher and 2.12.14 and higher)