Am Donnerstag 24 September 2009 00:03:03 schrieb Michael Mossey:
Trying to understand Douglas Auclair's article "MonadPlus - What a super monad!" here http://www.haskell.org/sitewiki/images/6/6a/TMR-Issue11.pdf
Defines
splits :: Eq a => [a] -> [(a,[a])] splits list = do x <- list return (x, delete x list)
choose :: Eq a => StateT [a] [] a choose = StateT (\s -> splits s)
I'm trying to understand what "StateT [a] [] a" means:
StateT stateType innerMonad resultType The state is a list of a, the inner monad is the list monad, apart from the newtype wrapper, it's [a] -> [(a,[a])]
I wrote
t1 :: StateT [Int] [] [Int] t1 = do s <- get return s
That compiles. Then I tried to write
t2 :: StateT [Int] [] [Int] t2 = do x <- [1,2,3] s <- get return (x:s)
I thought this would be fine because [1,2,3] is an example of a list monad.
Within one do-block, you can use only one monad. do x <- action1 y <- action2 return (f x y) desugars into action1 >>= (\x -> (action2 >>= (\y -> (return (f x y))))) (I used more parentheses than necessary to make the associativity clear, with less clutter, it's action1 >>= \x -> action2 >>= \y -> return (f x y) .) The type of (>>=) is (>>=) :: Monad m => m a -> (a -> m b) -> m b so you can use only one monad in such an expression. With StateT (most monad transformers, I think), you can achieve what you want with t2 :: StateT [Int] [] [Int] t2 = do x <- lift [1,2,3] s <- get return (x:s) 'lift' lifts actions in the inner monad to StateT actions.
But I get
"Can't match expected type StateT [Int] [] t against inferred type [a]"