# can this be written more easily?

Dean Herington heringto@cs.unc.edu
Sat, 22 Feb 2003 18:39:56 -0500 (EST)

```Ah, I see.  Finite maps might be appropriate, then.  Here's a sketch of
how you might implement arraylike objects with finite maps.

-- Array-like objects built from FiniteMaps
-- Dean Herington, Feb. 22, 2003

module FMArray (FMArray, fmArray, get, set, update) where

import Array
import FiniteMap
import Maybe

data FMArray i e = FMArray (i,i) (FiniteMap i e)

fmArray :: (Ix i) => (i,i) -> [(i,e)] -> FMArray i e
fmArray bs as =
if all (inRange bs . fst) as
then FMArray bs \$ listToFM [ (i, elemAt i) | i <- range bs ]
else outOfRange "fmArray"
where elemAt i = case [ e | (j,e) <- as, i == j ] of
[e] -> e
[]  -> error "FMArray: undefined element"
_   -> error "FMArray: multiply defined element"

get :: (Ix i) => FMArray i e -> i -> e
get (FMArray bs fm) i =
if inRange bs i then fromJust (lookupFM fm i) else outOfRange "get"

set :: (Ix i) => FMArray i e -> i -> e -> FMArray i e
set (FMArray bs fm) i e =
if inRange bs i then FMArray bs (addToFM fm i e) else outOfRange "set"

update :: (Ix i) => FMArray i e -> i -> (e -> e) -> FMArray i e
update (FMArray bs fm) i f =
if inRange bs i
then FMArray bs (addToFM_C (\ e _ -> f e) fm i undefined)
else outOfRange "update"

outOfRange who = error \$ "FMArray." ++ who ++ ": index out of range"

instance (Ix i, Show i, Show e) => Show (FMArray i e) where
showsPrec p (FMArray bs fm) =
showParen (p >= 11) (showString "FMArray " . shows bs .
showChar ' ' . shows (fmToList fm))

-- Dean

On Fri, 21 Feb 2003, Mike T. Machenry wrote:

> Eh, state is not possible. This is a recursive state space search. I need
> to branch the state of the game and not allow branches to effect others.
> Though I'd really like to represent them as arrays like such:
>
> data Player = Red | Green | Blue deriving (Enum,Eq,Ix)
> data Ticket = Taxi | Bus | Underground deriving (Enum,Eq,Ix)
> type Tickets = Array Player (Array Ticket Int)
>
> Arrays in Haskell just might be too painful to to this with and I'll have to
> use lists. They'er easier to use, but I don't believe they match the data
> as closly.
>
> -mike
>
> On Fri, Feb 21, 2003 at 12:50:11PM -0500, Dean Herington wrote:
> >
> > While I largely agree with what Nils said, it does seem that arrays are a good match
> > for your application.  It is true, unfortunately, as you're discovering, that
> > mutable arrays are awkward in a pure functional language.  I think the most
> > appropriate way to deal with them would depend on the larger perspective of your
> > application's control structure.  If the array manipulation is necessarily
> > intermixed with input/output, for example, you might consider an essentially
> > imperative approach using `IOArray`s in the `IO` monad.  If the input/ouput and
> > array manipulation tend to alternate, `STArray`s in the `ST` monad might offer a
> > more functional approach.  If the amount of array manipulation is small, then the
> > approach you've shown so far might be entirely adequate.
> >
> > Dean
> >
> >
> > "Mike T. Machenry" wrote:
> >
> > >   I guess I figured that Arrays were the natural data type for the tickets
> > > since it has a fixed size and the elements all have a specific player
> > > associated with them. I am coming from a Scheme background so I am already
> > > very fluent in list manipulation. I'm not an imperative programer, so that's
> > > not really the problem. I just think that Arrays represent this data much
> > > better.
> > >
> > > -mike
> > >
> > > On Fri, Feb 21, 2003 at 01:00:30PM +0100, Nils Decker wrote:
> > > > "Mike T. Machenry" <dskippy@ccs.neu.edu> wrote:
> > > > > Hey Everyone,
> > > > >
> > > > >   I am having a hard time making a data structure that I can
> > > > >   incrimentally
> > > > > update. Mostly because dealing with arrays is so tough. Does anyone
> > > > > think I'm going about this completely the wrong way? This is what I
> > > > > have.
> > > > IMO there is normally no need to use arrays in haskell. You should
> > > > use lists instead, because it is much more easy to use recursion
> > > > over lists. Once you got the pattern, it feels like the natural
> > > > way in haskell.
> > > > It also helps to define more types than just one large type for everything.
> > > >
> > > > > data GameState = GameState {
> > > > >   dTickets   :: Array Player (Array Ticket Int),
> > > > >   fTickets   :: Array FugitiveTicket Int,
> > > > >   history    :: [Move],
> > > > >   dLocations :: Array Player Stop,
> > > > >   fLocations :: Set Stop
> > > > > }
> > > > >
> > > > > removeTicket :: GameState -> Detective -> Ticket -> GameState
> > > > > removeTicket s d t =
> > > > >   s { tickets = (tickets s) // [(d,[(t,((tickets s)!d!t - 1))])] }
> > > >
> > > > why not use
> > > >
> > > > data Ticket = Ticket Int {-value-} deriving (Eq, Show)
> > > > type Tickets = [Ticket]
> > > >
> > > > removeTicket :: Ticket -> Tickets -> Tickets
> > > > removeTicket _ [] = fail "not there"
> > > > removeTicket x (t:ts)
> > > >   | x == t    = ts
> > > >   | otherwise = t : (removeTicket x ts)
> > > >
> > > > Is there a reason, to have different fields and types for detectives?
> > > > data PlayerNames = MrX | Red | Green | Blue deriving (Eq,Show)
> > > > If MrX needs special treatment ( computed move or information shown
> > > > to the player ) you can patternmatch for MrX. For the usage of tickets
> > > > and the history of moves there should be no difference.
> > > >
> > > > There might even be no reason to have PlayerNames as instance of enum.
> > > > As i understand, you want to use succ(player) to find the next player
> > > > to move.  It can be easier to have a function that recurses over a
> > > > list of players to run one round.
> > > >
> > > > > This remove ticket function is just terrible and it's common for me to
> > > > > have to do operations like this. It's been hard to make this a
> > > > > function that I can pattern match on, because which piece of data is
> > > > > manipulated depends on the parmeter d (Detective)
> > > > Just split up the huge record and have tiny functions to deal with
> > > > every specific part of it
> > > >
> > > > Summary in a few words: Use many small functions instead of a few
> > > > big ones. Use lists instead of arrays. If you use arrays, first understand
> > > > why you can not use lists in that specific case. Learn to recurse
> > > > over lists! Learn to use map, foldl and foldr. They save you a lot of
> > > > typing and make most functions dealing with lists short and clear.
> > > >
> > > > You should derive Show for all your types and test every new function
> > > > in hugs or ghci.
> > > >
> > > > Regards
> > > >   Nils Decker
> > > >
> > > > PS: I have learned haskell a year ago after using imperative languages
> > > >     all of my life. At first it is hard to get used to some concepts,
> > > >     but then they are wonderful.
> > > >
> > > > PPS: There is another list called haskell-cafe. It is used for discussion
> > > >      of problems while this list is meant for short threads and announcements.
> > > >      You might want to subscribe to it.
> > > >
> > > >
> > > > --
> > > > Freedom of speech is wonderful - right up there with the freedom
> > > > not to listen.
> > > >
> > > > Nils Decker <nils@ndecker.de>
> >
> > _______________________________________________