[Haskell-beginners] Custom type classes

[Daniel Hinojosa]

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.
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