[Haskell-beginners] Custom type classes

[Daniel Hinojosa]

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 at 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.
>
>
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