Wildcard Arguments in Types
The syntax of wildcard arguments in types has changed from _
to ?
. Example:
List[?]
Map[? <: AnyRef, ? >: Null]
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 makes higher-kinded types easier to use. It also removes 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:
- In Scala 3.0, both
_
and?
are legal names for wildcards. - In Scala 3.1,
_
is deprecated in favor of?
as a name for a wildcard. A-rewrite
option is available to rewrite one to the other. - In Scala 3.2, the meaning of
_
changes from wildcard to placeholder for type parameter. - The Scala 3.1 behavior is already available today under the
-strict
setting.
To smooth the transition for codebases that use kind-projector, we adopt the following measures under the command line
option -Ykind-projector
:
- In Scala 3.0,
*
is available as a type parameter placeholder. - In Scala 3.2,
*
is deprecated in favor of_
. A-rewrite
option is available to rewrite one to the other. - 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 option -Ykind-projector
.