Return specific type within Haskell

Question

I have a pretty general question about Haskell\'s type system. I\'m trying to become familiar with it, and I have the following function:

getN :: Num a =>         


        

Answer 1

Check out Existentially quantified types.

One way to solve it would be to define a new data type

data NumBox = forall n. Num n => NumBox n

You'll need -XExistentialQuantification to get this to work.

Now you can write something like

getN :: NumBox
getN = NumBox (5.0 :: Double)

You can also define a NumBox-list as

let n3 = [NumBox (4.0 :: Double), NumBox (1 :: Integer), NumBox (1 :: Int) ]