Creating a list of Ints and functions Int -> Int -> Int

Question

Apart from creating functions that do simple things to lists, I'm pretty new to haskell. I would like to create a list which contains things of type Int, and functions of type Int -> Int -> Int.

Here is what I have tried:

data Token = Value Int | Operator (Int -> Int -> Int)

tokens :: [Token]
tokens = [12, (+)]

but I get the following error

Couldn't match expected type `Token'
            with actual type `Integer -> Integer -> Integer'
In the expression: (+)
In the expression: [12, (+)]
In an equation for `tokens': tokens = [12, (+)]

I'm not sure why this doesn't work, can anyone point me in the right direction?

Answer 1

You need to use your constructors to obtain values of type Token. For example, 12 is not of type Token, it is of type Int (well, Num a => a). Similarly, (+) is not a token but a function Int -> Int -> Int. Notice that Token /= Int -> Int -> Int.

Fortunately you have defined a few constructors such as Value :: Int -> Token and Operator :: (Int -> Int -> Int) -> Token. So using those we get:

tokens :: [Token]
tokens = [Value 12, Operator (+)]