On 2008 Nov 1, at 12:23, Larry Evans wrote:
That still leaves me confused about:
put (n+1) AFAICT, with the preceding |n <- get|, this means:
put (get+1)
The <- syntax (actually (>>=)) "unwraps" a monadic value (on condition that it is "rewrapped" later), so instead of |State Int Int| it's just |Int|.
I'm trying to understand how to use MonadState. I would have expected tick would have been defined:
tick = State (\n -> (n+1,n+1))
I think you meant \n -> (n,n+1)
instead of the more obscure:
tick = do n <- get put (n+1) return n
The above translates to get >>= \n -> put (n+1) >> return n Expand (>>=): State (\s -> let (a,s') = runState get s in runState (put (a+1)) s') Expand |get| and |put|: State (\s -> let (a,s') = runState (State (\s'' -> (s'',s''))) in runState (State (\_ -> ((),a+1))) s') |runState| and |State| cancel: State (\s -> let (a,s') = (\s'' -> (s'',s'')) in (\_ -> (s',a+1))) Now expand the inner lambdas: State (\s -> let (a,s') = (s,s) in (s',a+1)) And collapse the let: State (\s -> (s,s+1)) (I think I did that right; at least I got the right result...) The point of using the "more obscure" definition is that it uses the machinery already defined for |State| --- and (given that I'm not absolutely certain I expanded the above right!) using the machinery is easier to keep track of than expanding directly, while demonstrating the operation of |State| instead of just presenting a result without explanation of how it follows from the definition of |State|. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH