implicitNotFound
To customize the error message that's emitted when an implicit of type
C[T1,..., TN]
cannot be found, annotate the class C
with @implicitNotFound
.
Assuming C
has type parameters X1, ..., XN
, the error message will be the
result of replacing all occurrences of ${Xi}
in the string msg
with the
string representation of the corresponding type argument Ti
.
The annotation is effectively inherited by subtypes if they are not annotated.
The annotation can also be attached to implicit parameters. In this case, ${Xi}
can refer to type parameters in the current scope. The @implicitNotFound
message
on the parameter takes precedence over the one on the parameter's type.
import scala.annotation.implicitNotFound
@implicitNotFound("Could not find an implicit C[${T}, ${U}]")
class C[T, U]
class K[A] {
def m[B](implicit c: C[List[A], B]) = 0
def n[B](implicit @implicitNotFound("Specific message for C of list of ${A} and ${B}") c: C[List[A], B]) = 1
}
object Test {
val k = new K[Int]
k.m[String]
k.n[String]
}
The compiler issues the following error messages:
Test.scala:13: error: Could not find an implicit C[List[Int], String]
k.m[String]
^
Test.scala:14: error: Specific message for C of list of Int and String
k.n[String]
^
- Source:
- implicitNotFound.scala