I am trying to define a data type for storing RGB values
data Color = RGB Int Int Int
black = RGB 0 0 0
However since parameters always lie in range [0-255], is there a way to constrain arguments to data constructor such that value is always a valid color?
As the comments indicate, you can use Word8, which has the desired range.
import Data.Word(Word8)
data Color = Color Word8 Word8 Word8
Word8 implements Num so you can use regular integer constants with it.
The more general solution to the problem "I have a constraint that can't be expressed with types" is to hide your constructor and make a checked interface. In your case, that would manifest as
data Color = Color Int Int Int -- Don't export the constructor
color :: Int -> Int -> Int -> Maybe Color
color r g b
| r < 256 && g < 256 && b < 256 = Just (Color r g b)
| otherwise = Nothing
Color (upper case), call the constructor in the datatype something else, and don't export it, as the answer says.I'm unsure if it will work like this. but with some typeclass arithmetic it might be possible
Word8instead ofInt.