在Haskell中定义多态常量

Question

I've read about polymorphic constants/nullary polymorphic functions in Learn You A Haskell . It gave several examples of built-in ones, such as:

ghci> 20 :: Float  
20.0  
ghci> 20 :: Int  
20  
ghci> minBound :: Int  
-2147483648  
ghci> maxBound :: (Bool, Int, Char)  
(True,2147483647,'\1114111')  

However, it does not explain how to define your own. How are they defined?

Answer 1

You need to make a typeclass including the functions/constants you want, each with a variable return type. Instantiate it for each type you want your constants to be able to be.

class MyConstants a where
  one :: a
  ten :: a

instance MyConstants Int where
  one = 1
  ten = 10

instance MyConstants Float where
  one = 1.0
  ten = 10.0

instance MyConstants String where
  one = "one"
  ten = "ten"

Example Usage ( codepad )

main = do
  putStrLn . show $ (ten :: Int)
  putStrLn . show $ (one :: String)
  putStrLn . show $ (ten :: Float) + one
  putStrLn . show $ "Count from " ++ one ++ " to " ++ ten

10
"one"
11.0
"Count from one to ten"

Answer 2

First, I recommend against using the term "constant" to mean non-functions, since all values are constant (immutable), whether those values are functions (ie have function type) or not.

You don't even need type classes to have polymorphic non-functions. An example is [] . To define your own polymorphic non-functions, you can define a data type (as in the list example) or construct something out of already-defined pieces. For instance: empties = ([],[[]]) .