Let's say I have the following data model, for keeping track of the stats of baseball players, teams, and coaches:

data BBTeam = BBTeam { teamname :: String, 
                       manager :: Coach,
                       players :: [BBPlayer] }  
     deriving (Show)

data Coach = Coach { coachname :: String, 
                     favcussword :: String,
                     diet :: Diet }  
     deriving (Show)

data Diet = Diet { dietname :: String, 
                   steaks :: Integer, 
                   eggs :: Integer }  
     deriving (Show)

data BBPlayer = BBPlayer { playername :: String, 
                           hits :: Integer,
                           era :: Double }  
     deriving (Show)

Now let's say that managers, who are usually steak fanatics, want to eat even more steak -- so we need to be able to increase the steak content of a manager's diet. Here are two possible implementations for this function:

1) This uses lots of pattern matching and I have to get all of the argument ordering for all of the constructors right ... twice. It seems like it wouldn't scale very well or be very maintainable/readable.

addManagerSteak :: BBTeam -> BBTeam
addManagerSteak (BBTeam tname (Coach cname cuss (Diet dname oldsteaks oldeggs)) players) = BBTeam tname newcoach players
  where
    newcoach = Coach cname cuss (Diet dname (oldsteaks + 1) oldeggs)

2) This uses all of the accessors provided by Haskell's record syntax, but it is also ugly and repetitive, and hard to maintain and read, I think.

addManStk :: BBTeam -> BBTeam
addManStk team = newteam
  where
    newteam = BBTeam (teamname team) newmanager (players team)
    newmanager = Coach (coachname oldcoach) (favcussword oldcoach) newdiet
    oldcoach = manager team
    newdiet = Diet (dietname olddiet) (oldsteaks + 1) (eggs olddiet)
    olddiet = diet oldcoach
    oldsteaks = steaks olddiet

                    
                    

                        
                        haskell
                        
                        functional-programming
                        
                        clojure
                        
                        static-typing
                        
                    

                    

                        
                        
                        
                        
                        
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