Question

I’m putting together a Traveller implementation, and have begun by defining my data structures. I ran into a problem when trying to define a Ship.
I began with some simple data definitions.

data Ship = Ship Cargo Hull Weapons Engines

data Cargo = WholeMagilla
           | MostOfIt
           | HalfOfIt
           | SomeOfIt

data Hull = Heavy
          | AboveAverage
          | Average
          | Meh

data Weapons = WarMonger
             | BadMofo
             | CautiousCarl
             | Pacifist

data Engines = WarpSuperFast
             | WarpFairlyFast
             | WarpFast
             | Turtle

Now here’s my problem. I want to restrict what values a type can be, based on what the values of the other types are. Example : A possible Ship could be

Ship WholeMagilla Heavy Pacifist Turtle
Ship WholeMagilla Meh   WarMonger Turtle
Ship WholeMagilla Meh Pacifist WarpSuperFast

So, if a Player has enough Credits they can have two types of maximum value, at best, at the cost of minimizing the rest. Then, there are all the possibilities in between. I started visualizing a graph whose paths are determined by what nodes are already in that path. This helps me think about the problem, but not in a way that is going to help me write a function that will get the result I want. Could someone point me in the right direction?

Answer 1

You could associate “credits” to your objects and use a smart constructor. Then, wrap everything in a module and export only the smart constructor, not the Ship constructor, so users will not mistakenly use it.

Here is the code (I’ve stripped down your constructors, for simplicity):

data Ship = Ship Cargo Hull deriving Show  

class Credit a where                                                       
    credit :: a -> Int   

instance Credit Cargo where
    credit WholeMagilla = 1        
    credit MostOfIt = 2

instance Credit Hull where 
    credit Heavy = 1       
    credit AboveAverage = 2       

data Cargo = WholeMagilla                                                  
           | MostOfIt deriving Show

data Hull = Heavy                                                          
          | AboveAverage deriving Show

max_credit :: Int
max_credit = 3

ship :: Cargo -> Hull -> Ship
ship c h                                                                   
    | credit c + credit h < max_credit = Ship c h 
    | otherwise = error "Too many credits"  

main :: IO ()                                                              
main = do                                                                  
    print $ ship WholeMagilla Heavy     
    print $ ship WholeMagilla AboveAverage