I am pretty sure I have a good handle on type classes, but this one is perplexing me in Haskell. I created custom Tuples called Tuple2 and Tuple3 (I know Haskell already has Tuples, just thought I would create my own for this exercise). Then I wanted to create a type class that would have a method called first that would get the first element of the tuple regardless of what kind of Tuple it is. Tuple2, Tuple3, etc.

Here is what I have:

data Tuple3 a b c = Tuple3 a b c deriving (Show)

data Tuple2 a b = Tuple2 a b deriving (Show)

class Indexable idx where

first :: idx c -> a

instance Indexable (Tuple2 a) where

first (Tuple2 a b) = a

In my main, I try to get call putStrLn $ show $ first $ Tuple2 1 "One"

I was greeted with the following trace:

Couldn't match expected type ‘a1’ with actual type ‘a’

‘a’ is a rigid type variable bound by

the instance declaration at TypeClasses.hs:35:10

‘a1’ is a rigid type variable bound by

the type signature for first :: Tuple2 a c -> a1

at TypeClasses.hs:36:4

Relevant bindings include

a :: a (bound at TypeClasses.hs:36:18)

first :: Tuple2 a c -> a1 (bound at TypeClasses.hs:36:4)

In the expression: a

In an equation for ‘first’: first (Tuple2 a b) = a

Help is appreciated.