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. :-) Many thanks in advance, Martijn. [1] http://hackage.haskell.org/package/xformat [2] http://code.google.com/p/monoid-cont/source/browse/trunk/ContSt.hs