[Haskell-cafe] Do we have idiom for lifting a state monad into pair of states?

[Roman Cheplyaka]

monad-classes can do this, and more: http://bit.ly/1xBZr3I

On 26/03/15 17:32, lennart spitzner wrote:
> Let me grab this opportunity to advertise the multistate package [1].
> 
> The underlying assumption is that the types of the states in your
> stack are distinct. In that case, type inference can give you
> something like "automatic zooming" for a MultiState that contains
> arbitrary heterogenous lists (tuples).
> Generous usage of the Monad(Multi)State type class would thereby
> eliminate the need for do1st and do2nd. See the example in the package.
> 
> The package does not (yet!) work well with existing transformers, for
> example one might think of running a StateT on one of the states in a
> MultiStateT. This is not a problem when using MultiStateT exclusively,
> but of course bad for interoperability. I am open for specific
> requests in that direction.
> 
> In the last few days I have been working on adding a MultiRWST, but
> this is not completely finished yet (and I am not sure yet how to make
> the interface consistent; the whole run/eval/exec distinction seems
> unintuitive, especially when you have multiple states..)
> 
> Lennart
> 
> 
> [1] https://hackage.haskell.org/package/multistate
> 
> 
> On 26/03/15 09:28, Ki Yung Ahn wrote:
>> Consider you have developed library routines that act on (State s1 a).
>> For some reason, you need more than one state simultaneously. Let's
>> say two side by side for simple example, that is (State (s1,s2) a). To
>> use library functions on one state monad in a two state monad, we need
>> to wrapper that lifts actions of (State s1 a) to (State (s1,s2) a).
>>
>> It is not difficult to write a lifter for such purposes as below. It
>> is kind of like doing liftA in Applicative libarary, but instead of
>> the last argument 'a' but on the fist argument 's' of (State s a).
>> This seems like an idiom that can often come up. So, I tried some
>> searching in some applicative related libraries and monad transformer
>> related libraries but haven't found this idiom yet.
>>
>> If you had a need for idioms like below, what do you call it? Or, is
>> there a way more general way I've missed that makes this a very
>> special case of it.
>>
>>> import Control.Monad.State
>>> import Control.Monad.Error
>>> import Control.Applicative
>>>
>>> -- lift an action over a state into a pair of states
>>> --
>>> do1st :: State s1 a -> State (s1,s2) a
>>> do1st m1 = do (s1, s2) <- get
>>>               let (a, s1') = runState m1 s1
>>>               put (s1',s2)
>>>               return a
>>>
>>> do2nd :: State s2 a -> State (s1,s2) a
>>> do2nd m2 = do (s1, s2) <- get
>>>               let (a, s2') = runState m2 s2
>>>               put (s1,s2')
>>>               return a
>>>
>>>
>>> -- lift an action over a state with error
>>> -- into a pair of states with error
>>> --
>>> do1 :: Error e => ErrorT e (State s1) a -> ErrorT e (State (s1,s2)) a
>>> do1 m1 = do (s1, s2) <- lift get
>>>             let (ma,s1') = (runState . runErrorT) m1 s1
>>>             case ma of
>>>               Left e  -> throwError e
>>>               Right a -> do lift $ put (s1',s2)
>>>                             return a
>>>
>>>
>>> do2 :: Error e => ErrorT e (State s2) a -> ErrorT e (State (s1,s2)) a
>>> do2 m2 = do (s1, s2) <- lift get
>>>             let (ma,s2') = (runState . runErrorT) m2 s2
>>>             case ma of
>>>               Left e  -> throwError e
>>>               Right a -> do lift $ put (s1,s2')
>>>                             return a
>>
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> 
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