Re: [Haskell-cafe] seems like I'm on the wrong track

2 Dec 2009


      Am Mittwoch 02 Dezember 2009 03:28:04 schrieb Michael Mossey:
...
Daniel Fischer wrote:
...
...
getNumber :: String -> AssignedNumbers -> (Int,AssignedNumbers)
Yeah, that screams State Monad.
Hi, thanks for all the advice.
I was hoping my AssignedNumbers "class" would be useful with many data
structures. In other words I would have
data State1 = State1 { a :: AssignedNumbers, b :: AssignedNumbers, ... }
data State2 = State2 { c :: AssignedNumbers, d :: AssignedNumbers, ... }
func1 :: State State1 Int
func1 = do
    ... something using a and b ...
func2 :: State State2 Int
func2 = do
    ... something using c and d ...
So I thought maybe I could defined a function like
nextNumber :: MonadState s m => (s -> AssignedNumbers) -> (AssignedNumbers
-> s) -> m Int
nextNumber retreive putBack = ...
and have it be useful in both "State State1 a" and "State State2 a" monads,
but defining the retrieve and putBack functions isn't pretty.
I will try to grok Robert's reply also. Maybe he has something addressing
this.
Definitely.

data AssignType = Oscillator | Table | Thingamajig
    deriving (Eq, Ord, Ix)

data MusicState
    = MS
    { assignedNumbers :: Array AssignType AssignedNumbers
    , other stuff
    } deriving (Everything you need)

fetchNumber :: String -> AssignType -> State MusicState Int
fetchNumber str ty = do
    an <- gets ((! ty) . assignedNumbers)
    let (i,newAn) = getNumber str an
    modify (\ms -> ms{ assignedNumbers = assignedNumbers ms // [(ty,newAn)] })
    return i

doSomeMusicStuff aString1 aString2 = do
    o <- fetchNumber aString1 Oscillator
    t <- fetchNumber aString2 Table
    return (o,t)
...
Mike