constant variable declaration in Haskell

Question

To declare constant variables I can do following in Ruby

class COLOR
   RED   = 10
   BLUE  = 20
   GREEM = 30
end

COLOR::RED returns 10 , COLOR::BLUE returns 20 , and so on. How do I accomplish that in Haskell?

I want to have a namespace name in front of my variable name. Maybe the example above is not a good example. For the case below, you can see including a namespace name will make a variable much easier to understand.

class BASEBALL_TEAM
   GIANTS = 15
   METS = 30
   REDS = 45
   ...
end

BASEBALL_TEAM::GIANTS is much clear than GIANTS .

based on the comments below, it seems the only way I can accomplish it is by doing something like below:

module Color (Color) where
data Color = Red | Blue | Green deriving (Eq, Show, Ord, Bounded, Enum)                        

fromEnum' x = (fromEnum x) + 10

to get integer value of 10 for Color.Red , I have to write fromEnum Color.Red , the syntax is not very clean.

Answer 1

Untagged constants are bad. If you go with bunch of Int constants then you lose type-checking (think about possible values that Int -> whatever function takes as opposed to SomeConstType -> whatever ) and possibly introduce bugs. You want a strong type instead:

data Colour = Red | Blue | Green deriving (Show, Eq, Enum)

Also, representing those values as integers is not really necessary in most cases. If you actually do need it, Enum typeclass provides toEnum and fromEnum .

As for namespaces: modules are namespaces in Haskell. Put your type in a module and then you can import it qualified and you'll have your prefix:

-- Colours.hs
module Colours (Colour) where
data Colour = ...

-- SomeOtherModule.hs
module SomeOtherModule where
import qualified Colours

foo = Colours.Red

That said, creating modules just for this one type with constants (or importing them qualified) is not really necessary, because you can easily trace things by their types.

Answer 2

Things are constant in Haskell by default, so

red = 10
blue = 20
green = 30

would be equivalent.

A more interesting question would be why you want to do this? There are likely better ways to accomplish what you want in Haskell. The answer by @CatPlusPlus shows a good way of doing this.

This seems like a very basic Haskell question, so I'll politely point you to Learn you a Haskell , which, in my opinion, is the best resource to get started with Haskell.

Another promising resource for learning Haskell is FP complete's School of Haskell , which is currently in beta, but I haven't tried it myself. This is a more interactive setting, where you can directly try things out in the browser.