On 8/17/07, Andrew Coppin
I've seen comments in various places that monads allow you to "borrow things from the future".
That sounds completely absurd to me... can anybody explain?
Suppose you buy into the notion that monads sequence actions. Then consider the following code:
import Control.Monad.State
test = do put $ x+1 x <- return 1 return undefined
go = execState test undefined
execState runs a sequence of actions in the state monad, ignoring the returned value and simply giving you back the resulting state. So work through what this code does: It sets the value of the state to be 1+x. It then sets x to be 1. And then it returns undefined. We don't care about the return value, we just care about the state. And clearly the state is 2. But if you believe all that action sequencing stuff, we set the state using x before we actually set the value of x. So we're reading from the future. But you can breathe a sigh of relief because the above code doesn't compile and the laws of physics remain unharmed. Except...you can switch on ghc's special time travel features using the -fglasgow-exts flag. Use the time-travel compatible mdo instead of do and you'll find that this compiles and runs fine:
import Control.Monad.State import Control.Monad.Fix
test = mdo put $ x+1 x <- return 1 return undefined
go = execState test undefined -- Dan