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One of the great limitations of the cake pattern is that its static. I would like to be able to mix-in traits potentially written by different coders completely independently. However the traits would not need to be mixed-in frequently. The user would have an initialisation screen where they would choose the traits / assemblies, before the main application was run. So the thought occurred to me why not mix-in and compile the chosen traits from with in the user choice selection module. If the compilation failed, no problem the user would just get back some message - incompatible assemblies or what ever. If the compilation succeeded then the top UI module would load the newly compiled classes with the pre-compiled parts of the assemblies and run the main application. Note there might only need to be one or two classes compiled duruing run time initialisation. All the rest of the code could have been compiled normally.

I'm pretty new to Scala. Is this a recognised pattern? Is there any support for it? It seems mad to have to use Guice for a relative simple dependency situation. Can I run the Scala compiler easily from within an application? Can I run it in memory and its outputs be used from memory without unnecessary file creation?

Note: Although appearing to be dynamic, this methodology would remain 100% static.

Edit it occurs to that one of the drives of Microsoft's Roslyn project was to enable just this sort of thing for C# and Visual Basic. But that seems to have been a pretty big project even for a high powered Microsoft team.

Rich Oliver
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  • There was a question in past few days, that looks like yours: http://stackoverflow.com/questions/11054299/can-we-assign-change-traits-to-the-scala-class-during-runtime-how-any-sampl – om-nom-nom Jun 16 '12 at 14:47
  • @om-nom-nom No I don't think its the same question. I'm merely trying to automate what a developer could do manually. Select some mixins compile the classes, make a module combine it with the pre existing libraries and then run the programme. It doesn't go against the static paradigm at all. – Rich Oliver Jun 16 '12 at 14:51
  • @RichOliver The code in those traits have to be inserted into a class an compiled for it to work, given JVM's limitations. – Daniel C. Sobral Jun 17 '12 at 01:42

1 Answers1

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Calling the compiler directly from within Scala is doable, but not for the timid. Luckily, the good people at Twitter have automated the process for you. (140 character celebrity micro-blogging, and some cool Scala utilities! Thanks Twitter.) You can use the com.twitter.utils.Eval class to compile and evaluate Scala strings. In your example, you would do something like

val eval = new Eval()
val myObj = eval[BaseClass]("new BaseClass extends " + traitNameList.mkString(" with "))

This will create you a new object with all of the traits you desire built in. The question then arises as to whether this is a good idea. Downsides:

  • Calling out to the Scala compiler is not quick
  • If you do this enough, you will overload the PermGen space, as the classes you create will never be garbage collected
  • This really is more of the sort of thing you want a dynamic language for rather than Scala. You're likely to find places where this all kinds of works, but clashes with the rest of your architecture (yes, that's vague).
Dave Griffith
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  • Yes I guess I was optimistically looking for compiler as a service, which has proved a pretty major project even for the C# team. – Rich Oliver Jun 16 '12 at 15:20
  • Given the context in the question details, none of the downsides above appear to be show-stoppers. – Robin Green Jun 17 '12 at 10:19