Re: [Haskell-cafe] StateWriter: a monad-writing exercise

On Thu, 25 Sep 2008, Anthony LODI wrote:

Hello haskell-cafe,

In my application I have a complex state threaded through long computation chains, and I need a way to log all state changes (so that the evolving state can be animated/replayed somewhere else). Initially I tried combining State and Writer monads but this allows for the possibility to change the state, and forget to write a log entry, etc.

Instead of rewriting from scratch, you can also define newtype StateWriter l s a = StateWriter (StateT s (Writer l) a) and then the monad instance is just about wrapping and unwrapping. Or you use {-# LANGUAGE GeneralizedNewtypeDeriving #-} and 'deriving Monad'.