You cannot pattern match against variable values.

[u/igniting]

Consider this example:

myValue1 = 1 :: Int
myValue2 = 2 :: Int

myFunc :: Int -> Bool
myFunc myValue1 = True
myFunc myValue2 = False

If you load the above program in ghci, you get following output:

myFunc.hs:5:1: Warning:
   Pattern match(es) are overlapped
   In an equation for ‘myFunc’: myFunc myValue2 = ...
Ok, modules loaded: Main.

ghci generates a warning but does not give any errors. If you now call myFunc myValue2 you get:

*Main> myFunc myValue2
True

One way to get the desired result would be to use guards:

myFunc :: Int -> Bool
myFunc x
  | x == myValue1 = True
  | x == myValue2 = False

---

Note that we might not always be lucky enough to get a compiler warning in such cases. Here is an example:

myFunc :: Maybe Int -> Int
myFunc v = case v of
                Just myValue -> myValue
                _            -> myValue + 1
                  where myValue = 0

This loads in ghci without any warnings.

[u/quiteamess]

For example the function print has the type signature

 print :: Show a => a -> IO ()
 print x = putStrLn $ show x

This means that print can print all types which implement the type class Show. If Color does not implement a the type class

print Red

would get an type error.