Annotate your type classes

This is a real quick post saying that as of 2.8.1, everyone should be annotating their type class traits for better error messages. Obviously, those of us who are supporting 2.8.0 or earlier are left out.

Here's an example REPL session:

Welcome to Scala version 2.9.0.r24384-b20110305105029 (OpenJDK 64-Bit Server VM, Java 1.6.0_20).
Type in expressions to have them evaluated.
Type :help for more information.

scala> import annotation.implicitNotFound
import annotation.implicitNotFound

scala> @implicitNotFound(msg = "Cannot find Serializable type class for ${T}") trait Serializable[T]
defined trait Serializable

scala> def foo[X : Serializable](x : X) = x
foo: [X](x: X)(implicit evidence$1: Serializable[X])X

scala> foo(5)
:11: error: Cannot find Serializable type class for Int

The annotation has very little documentation, but apparently supports ${TypeName} templates to inject the names of types the compiler is looking for. I believe you can also use this implicit on types with no type parameters.

Happy hacking everyone!


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