Scala: Delimited Continuations Explained - Not

问题:

Interested in the concept of continuation, I started reading wikis, posts, and came to this "simple" example:

reset {
  ...
  shift { k: (Int=>Int) =>  // the continuation k will be the '_ + 1' below
    k(7)
  } + 1
}
// result: 8

Without knowledge of Scala, I'm totally lost here, could not figure out how the 8 comes out.

Below is how I tried to figure out the meaning but failed. Any guy could you please give me a short explanation? Yeah there are Scala grammar books but they are too thick, I'm more interested in understanding delimited continuation concept than master Scala language...

• (Int=>Int)

  Looks like a C# delegate, input is Int, output is Int.

• k: (Int=>Int) => k(7)

  I'm lost here... what is k, and what is (Int=>Int)=>k(7)?

• shift { k: (Int=>Int) => k(7) } + 1

  Even more lost...

• reset { ...; shift { k: (Int=>Int) => k(7) } + 1 }

  Even even more and more lost...

回答1:

I found Chris League's talk, "Monadologie: Professional Help for Type Anxiety" (http://vimeo.com/13304075) contains one of better examples of a delimited continuation.

回答2:

fotNelton, thanks a lot! Alex Neth's answer in the link helped me out. Now I think I got it. Let me answer my own question to dot down as a note.

Grammar of reset and shift:

reset {
  ...
  shift { cf: (InputParameterType => OutpututParameterType) =>
     CodeBlockInsideShift
  } 
  CodeBlockAfterShiftBeforeEndOfReset
}

It actually means, in C# style pseudo code:

public delegate OutpututParameterType CFDelegate(InputParameterType);

CFDelegate cf = CodeBlockAfterShiftBeforeEndOfReset;

CodeBlockInsideShift;