- About Scala
- Documentation
- Code Examples
- Software
- Scala Developers
2.8 REPL power commands: reference material?
Tue, 2010-08-10, 12:02
Is there somewhere I could look to get more details about the myriad of new "power" commands in the REPL? I'm interested in a) What is (for example) power.mkType useful for? and; b) How do I use power.mkType?
Googling around, I pulled up (what looks like) an out-of-date blog posting from 2009 (http://blog.xebia.com/2009/07/19/scala-repl-tips-and-tricks-trunk-only/). I say OOD, because it runs through snippets like this one:
scala> mkType("IntList", "List[Int]")defined type alias IntListres3: scala.tools.nsc.InterpreterResults.Result = Success
which has changed in 2.8 final to look like:
scala> power.mkAlias("IntList","List[Int]") defined type alias IntListres8: scala.tools.nsc.InterpreterResults.Result = Success
~~~
kesh. mmm...chocolatey.
Googling around, I pulled up (what looks like) an out-of-date blog posting from 2009 (http://blog.xebia.com/2009/07/19/scala-repl-tips-and-tricks-trunk-only/). I say OOD, because it runs through snippets like this one:
scala> mkType("IntList", "List[Int]")defined type alias IntListres3: scala.tools.nsc.InterpreterResults.Result = Success
which has changed in 2.8 final to look like:
scala> power.mkAlias("IntList","List[Int]") defined type alias IntListres8: scala.tools.nsc.InterpreterResults.Result = Success
~~~
kesh. mmm...chocolatey.
On Tue, Aug 10, 2010 at 04:03:01AM -0700, Rakesh Karia wrote:
> Is there somewhere I could look to get more details about the myriad
> of new "power" commands in the REPL?
These are semi-intentionally undocumented at this point because there's
no safety net and it's still very in progress. Many results would seem
strange or wrong if you don't know a fair bit about the compiler phases,
and decent phase related tools aren't in there yet.
> I'm interested in a) What is (for example) power.mkType useful for?
> and; b) How do I use power.mkType?
In that specific case I can tell you what the motivation was: I wanted
to write a "typesafe printf" (in quotes because this is cheating a bit)
and I did so by translating the format string into a function type and
then applying the function to the other arguments. I used mkType (now
mkAlias I guess) to inject the function signature into the repl.
That name change is because the above involves only a type alias, but
mkType is for creating an instance of Type, which is the compiler's
internal representation for them. Let's see...
scala> :power
** Power User mode enabled - BEEP BOOP **
** scala.tools.nsc._ has been imported **
** New vals! Try repl, global, power **
** New cmds! :help to discover them **
** New defs! Type power. to reveal **
scala> import global._
import global._
scala> power.mkType("scala.collection.Map")
res0: power.compiler.Type = object scala.collection.Map
scala> res0.member("newBuilder")
res1: power.compiler.Symbol = method newBuilder
scala> res1.ownerChain
res2: List[power.compiler.Symbol] = List(method newBuilder, class MapFactory, package generic, package collection, package scala, package )
etc.
All that aside, the official answer is that these methods are documented
by the implementation and are only really intended for people who are
willing to examine said implementation, since you'll definitely have to
do that with the compiler to make much use of them.
So the docs: http://bit.ly/ahcKdz