Quick adjustment, playing around too much with it, that should be:

class Indexable idx where

   first :: idx a -> a

Problem still exists.

On Sun, Jan 24, 2016 at 11:25 PM, Daniel Hinojosa <dhinojosa@evolutionnext.com> wrote:

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.