Renaming classOf in Scala

Question

I'm working on a customer-readable DSL for ScalaTest. At the moment I can write

feature("Admin Login") {
  scenario("Correct username and password") {
    given("user visits", classOf[AdminHomePage])
    then(classOf[SignInPage], "is displayed")

but this would read a lot better as

feature("Admin Login") {
  scenario("Correct username and password") {
    given("user visits", the[AdminHomePage])
    then(the[SignInPage], "is displayed")

Is there any way to

def the[T] =

to return classOf[T] ?

Community · Accepted Answer · 2017-05-23 12:18:27Z

17

You could try this:

def the[T: ClassManifest]: Class[T] =
  classManifest[T].erasure.asInstanceOf[Class[T]]

The notation [T: ClassManifest] is a context bound and is equivalent to:

def the[T](implicit classManifest: ClassManifest[T])

Implicit values for Manifest[T] and ClassManifest[T] are automatically filled in by the compiler (if it can reify the type parameter passed to the method) and give you run-time information about T: ClassManifest gives just its erasure as a Class[_], and Manifest additionally can inform you about a possible parametrization of T itself (e.g., if T is Option[String], then you can learn about the String part, too).

edited May 23, 2017 at 12:18

CommunityBot

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answered Jun 9, 2011 at 9:14

Jean-Philippe Pellet

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3 Comments

Duncan McGregor Over a year ago

Thank you - I have tried it, and it works! I have no idea what it does though - I've gone from being a big Java shark to a Scala minnow ;-)

Jean-Philippe Pellet Over a year ago

@Duncan I've added a short explanation about context bounds and manifests.

Duncan McGregor Over a year ago

You're a gent - yet more Scala'y goodness to digest. I haven't been this keen since I bought Coplien's book on C++

Community · Accepted Answer · 2017-05-23 10:00:11Z

3

What you probably want to do is just rename the method (which is defined in the Predef object) on import:

import Predef.{ classOf => the, _ }

Note that classOf won't work anymore if you rename it like this. If you still need it, also add this import:

import Predef.classOf;

For more renaming goodness see also:

edited May 23, 2017 at 10:00

CommunityBot

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answered Jun 9, 2011 at 13:26

fresskoma

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5 Comments

Jean-Philippe Pellet Over a year ago

Nice, but this depends on clients of your library writing the right imports.

fresskoma Over a year ago

Well you'd have to import your own the implementation too, wouldn't you?

Jean-Philippe Pellet Over a year ago

Yes, with something easy like import mylib._, without the need for an extra import Predef.{ classOf => the, _ }.

Jens Schauder Over a year ago

yes but there is a big difference between telling your users 'just import xxx._' and 'import xxx._ and Predef.{ classOf => the, _ } and if you still need it Predef.classOf'

Jean-Philippe Pellet Over a year ago

@x3ro Good point about the automatic package import. I asked a similar question here, and for lack of answers I've concluded that such a thing is not possible at the time (2.9).

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