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'.
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