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

[Robert Greayer]

On Tue, Dec 1, 2009 at 8:01 PM, Michael P Mossey <mpm at alumni.caltech.edu>wrote:

> Perhaps someone could either (1) help me do what I'm trying to do, or (2)
> show me a better way.
>
> I have a problem that is very state-ful and I keep thinking of it as OO,
> which is driving me crazy. Haskell is several times harder to use than
> Python in this instance, probably because I'm doing it wrong.
>
> To give you a larger context, this problem is essentially compiling a
> description of music (my own) into a kind of music-machine-language
> (CSound). CSound is relatively untidy.
>
> In this one example, in a OO way of thinking, I have data called
> AssignedNumbers that assigns integers to unique strings and keeps track of
> the used integers and next available integer (the choice of available
> integer could follow a number of conventions so I wanted to hide that in an
> ADT.) So it has an associated function:
>
> getNumber :: String -> AssignedNumbers -> (Int,AssignedNumbers)
>
> What getNumber does is:
>
>  - check if the string already has a number assigned to it. If so, return
> that number.
>
>  - if not, pick the next available number.
>
>  - in all cases, return the possibly changed state of AssignedNumbers
>
> Then in a larger data structure, it contains fields of type
> AssignedNumbers. Like
>
> data MusicStuff = MusicStuff
>  { oscillatorNumbers :: AssignedNumbers
>  , tableNumbers :: AssignedNumbers
>  , ... }
>
> I'm using MusicStuff in a State monad, so I might write a function like
>
> doSomeMusicStuff :: String -> String -> State MusicStuff (Int,Int)
> doSomeMusicStuff aString1 aString2 = do
>   ms <- get
>   (o1,newOscNums) = getNumber aString1 (oscillatorNumbers ms)
>   (t1,newTabNums) = getNumber aString2 (tableNumbers ms)
>   put ms { oscillatorNumbers = newOscNums
>          , tableNumbers = newTabNums }
>   return (o1,t1)
>
> For what it does, this is extremely verbose and filled with distracting
> visual content. And this is just a very simple example---my real problem is
> several times more state-ful. Is there a better way?
>

As a quick observation, you might consider changing getNumber to be
something like:

nextNumber :: String -> NumberGroup -> State MusicStuff Int

where NumberGroup is something like

data NumberGroup = OscNums | TabNums |...

nextNumber updates the appropriate set of numbers in MusicStuff and returns
the number. doSomeMusicStuff then becomes:

doSomeMusicStuff aString1 aString2 = (,) `liftM` nextNumber OscNums `ap`
nextNumber TabNums

or better yet (applicatively)

doSomeMusicStuff aString1 aString2 = (,) <$> nextNumber OscNums <*>
nextNumber TabNums
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