Question

I would ideally like to write something like this:

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

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

Calling myFunc myValue2 returns True – not what I intend. I know why this happens, but is there a way to express this in Haskell without resorting to C-style #define statements?

Answer 1

Well, Haskell doesn’t unify names like this. Those new ‘myValue1’ and ‘2’ identifiers are new variables you’re binding.

The most Haskelly way is to use strong types and pattern matching:

data Values
   = D1
   | D2

myFunc :: Values -> Bool
myFunc D1 = True
myFunc D2 = False

Giving you a static guarantee only “1” or “2” can be passed to myFunc, proper symbolic matching and you even retain conversion to integers by deriving Enum.