On Mon, Sep 14, 2009 at 10:25 AM, Martijn van Steenbergen
Hello cafe,
Inspired by Sean Leather's xformat package [1] I built a datatype with which you can build a monoid with holes, yielding a function type to fill in these holes, continuation-passing style. Here are some primitives and their types:
now :: m -> ContSt m r r later :: (a -> m) -> ContSt m r (a -> r) run :: ContSt m m r -> r instance Monoid m => Category (ContSt m)
Here's an example of how to use it:
run (now "hello" . now "world")
"helloworld"
run (later id . now "world") "hello"
"helloworld"
run (later id . later show) "hello" 567
"hello567"
The source code is available at [2].
I have a couple of questions: * ContSt is a Category. Is it also an Arrow? Why (not)? * Did I miss any other obvious classes this type is an instance of? * What is its relation with the Cont and Reader monads? * Are there any other useful applications other than printf-like functionality? * ContSt is a horrible name. What is a better one?
For those who have a bit more time: I appreciate any comments and suggestions on the code. :-)
I believe this technique is based on a technique introduced in Olivier Danvy's "Functional Unparsing". While not immediately applicable to Haskell unless you want to make/use a delimited continuation monad, you may find the paper "On Typing Delimited Continuations: Three New Solutions to the Printf Problem" by Kenichi Asai interesting. It is available at the following url: http://pllab.is.ocha.ac.jp/~asai/papers/papers.html