Andres Löh wrote:
Hi everyone.
I'll follow Simon's lead, and ask a similar question with a similar motivation. I'm going to talk about free monads at the upcoming Haskell eXchange next Wednesday. I'll not limit myself to a particular library, and I'm open to related approaches (e.g. "operational") as well.
I'm also looking for as many compelling examples as possible. Like Simon, I don't want to know anything secret or anything that you wouldn't like me to include in my talk. Most useful are pointers to existing libraries using free monads that I might have missed (for example, because they're new or very specialized).
I have included a couple of examples with the operational package: https://github.com/HeinrichApfelmus/operational/tree/master/doc/examples#rea... My personal highlights are: BreadthFirstParsing.hs An implementation of parser combinators that does not have the space leak associated to the depth-first approach s -> [(a,s)] . Instead, parsers are evaluated in a breadth-first fashion, so that the input can be consumed in an on-line fashion. This is equivalent to Koen Classen's "parallel parsing processes", which I believe are used to implement the 'Text.ReadP' module. WebSessionState.lhs A program is written as a sequence of instructions, in this case a web session ("shopping cart checkout"). However, unlike the code may suggest, the program actually exits before completing and continues at the last instruction when started. No persistent state is kept in RAM. I learned this idea from Peter Thiemann's WASH/CGI library. TicTacToe.hs Similar to the previous example, but dialed up a notch. A game is implemented using a monad that returns player input. Internally, however, the player input can come from many sources: user, computer, or even replays, depending on how the free monad is interpreted. A less useful but still interesting example is `State.hs`, which implements the state monad using only s -> a instead of s -> (a,s) . Another example that I forgot to add when writing the package, but which also one of my highlights, is the probability monad. Using operational, it is possible to use the same code for both calculating complete probability distributions and sampling from said probability distributions. Yet another example for free monads/operational is the sunroof package http://hackage.haskell.org/package/sunroof-compiler, which is an EDSL that compiles to JavaScript. JavaScript statements are sequenced in a monad, but the monad is then interpreted as a syntax tree ("deep embedding"). Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com