Re: [Haskell-cafe] Re: ANNOUNCE: GotoT-transformers version 1.0

8 Sep 2010

      On 09/08/10 19:14, Ertugrul Soeylemez wrote:
...
Gregory Crosswhite  wrote:
...
People want to believe that Haskell is a better language than C, but
how could this possibly be true when Haskell lacks the very basic
"goto" feature???  If the world is going to take Haskell seriously,
then this serious blight needs to be addressed immediately!  Thus I
proud to present to you the "GotoT-transformers" package which
provides this missing functionality and so finally makes Haskell a
serious contender with C.
Have you looked at ContT from monadLib?  It's not just a goto, but in
fact a setjmp/longjmp, i.e. a goto with value.  I haven't used it for
anything yet, but it might come in handy for some algorithms:
import Data.List
  import MonadLib
  import Text.Printf
myComp :: ContT (Maybe Int) IO (Maybe Int)
  myComp = do
    (i, beginning) <- labelCC 0
    inBase $ printf "Current value: %i (type q to quit)\n" i
    query <- inBase getLine
    when ("q" `isPrefixOf` query) $ abort (Nothing :: Maybe Int)
    when (i < 10) $ jump (i+1) beginning
    return $ Just i
main :: IO ()
  main = runContT return myComp >>=
         printf "Final result: %s\n" . maybe "none" show
Greets,
Ertugrul
Whoa, that's cool!  I glanced at monadLib but I didn't realize that it
let you create labels that you could return to like that.  :-)  (I know
of callCC, but that isn't quite the same as this.)  Thanks for the pointer!

The limitation with continuation-based approaches to goto, though, is
that you can only jump back to points that you've seen before.  The
reason why I don't use a continuation-based approach in GotoT is because
I wanted the user (i.e., me, and maybe one or two other people if I'm
lucky :-) ) to be able to jump to an arbitrary point outside the
calculation that has never been visited before, rather than returning a
previously visited point of the same calculation.

Of course, if someone can prove to me that I am wrong and that GotoT
semantics can be implemented with continuations, then I would welcome
this information.  :-)

Cheers,
Greg