Compiler plugin for context encoding

[rssh]

Let's introduce context encoding for monadic operations, allowing programming in a colorless direct style.

We will represent M[X] as a context function:

     CpsMonadContext[M] ?=> X

The compiler plugin can transform types of functions that use CpsMonadContext[X] arguments (and functions that return such functions and … ), and in such a way, we can move all machinery with the monad brackets from the user code.

object network {
 def fetch(x:String): CpsMonadContext[Future] ?=> String =
   ???
}


def fetchList(urls: List[String]): CpsMonadContext[Future] ?=> List[String] =
   l.map(url => network.fetch(url) )

What's interesting is that runtime monadic reflection( which is mainstream in EPFL) uses the same encoding, so - a kind of transformation (runtime or compile-time cps transform) becomes an implementation detail that can depend on the monad type on platforms when it's supported.

Some problems:

• It is possible in the user code to transform the CpsMonadContext parameter into standard val and then pass it into other functions by using clause.
• Possible solutions:
  -- Using new capacity tracking functionality to make this impossible on type level (yet unclear how to do this)
  -- Check and block the assignment of monad-context in our compiler plugin.
  -- Extend the encoding to any function, not only the context function. (I.e., transform each function which will accept monad context).

Some possibilities:

• We can generate cps-transformed versions of HO functions. This is a complex thing, and cases that will come to cps-shifted objects exist, but 80% of simple cases can be built relatively simply.
• We can extend context encoding to have in the left part set of using effects instead CpsMonadContext, by adding the compiler plugin a layer, which will assemble our CpsMoandContext from used effects. An external effect stack, which can provide an effect list and appropriative monad, will be usable in context encoding.

The branch (unstable yet not working) is a compiler-plugin.

What are your thoughts in this direction?

[rssh]

released as @experimental future in 0.9.18

Changed from the original description: a context type is not a CpsMonadContext[F] but a new type CpsDirect[F], which use exactly for this purpose.

A few current issues before going to production:

• How to be sure that it is impossible to compile something with CpsDirect without the compiler plugin (?)
• When we have inclusion of G[] in F[]. can we call direct context function with CpsDirect[G] in F[_] (?)

[rssh]

Context inclusion is implemented.