[Haskell-beginners] Equality instance for lists

[Brent Yorgey]

On Sat, Nov 30, 2013 at 10:40:59PM +0000, Patrick Browne wrote:
>    Hi,
>    I am trying to write my own simplified class and instances for equality.
>    I have trouble with the equality equation for the empty list.
>    Even though I can use the [] == [] at the GHCi prompt I cannot use it in
>    my equality test.
>    How can I make my eq test handle empty lists while staying within the
>    context of my current code?.
>    Thanks,
>    Pat
> 
>    class Eq1 a where
>     eq :: a -> a -> Bool
>     
>    instance Eq1 Int where
>     eq a b =  a == b
> 
>    instance Eq1 a => Eq1 [a] where
>    -- This line compiles but gives an run time error.
>     eq [] [] =  True

This code does not give a run time error.  The error you are getting
is a type inference error, which has nothing to do with this code.

>     eq [] []

You simply need to give a type annotation on one of the empty lists.

  eq ([] :: [Int]) []

The standard Eq class has some built-in magic support by GHCI
(extended default rules) which means this does not apply in the case
of using == directly.

-Brent