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What is it?

The operational library makes it easy to implement monads with tricky control flow.

This is very useful for: writing web applications in a sequential style, programming games with a uniform interface for human and AI players and easy replay, implementing fast parser monads, designing monadic DSLs, etc.

For instance, imagine that you want to write a web application where the user is guided through a sequence of tasks ("wizard"). To structure your application, you can use a custom monad that supports an instruction askUserInput :: CustomMonad UserInput. This command sends a web form to the user and returns a result when he submits the form. However, you don't want your server to block while waiting for the user, so you have to suspend the computation and resume it at some later point. Sounds tricky to implement? This library makes it easy.

The idea is to identify a set of primitive instructions and to specify their operational semantics. Then, the library makes sure that the monad laws hold automatically. In the web application example, the primitive instruction would be AskUserInput.

Any monad can be implemented in this way. Ditto for monad transformers.

A thorough introduction to the ideas behind this library is given in "The Operational Monad Tutorial", published in Issue 15 of the Monad.Reader.

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