data Test = T1 | T2 | T3

class Eq1 a where
eq :: a -> a -> Bool

class Eq1 a => Ord1 a where
lt :: a -> a -> Bool

instance Eq1 Int where
eq a b = a == b

instance Eq1 Char where
eq a b = a == b

instance Eq1 Test where
eq a b = False

instance Ord1 Int where
lt a b = a < b

instance Eq1 a => Eq1 [a] where
eq [] [] = True
eq (x:xs) (y:ys) = eq x y && eq xs ys
eq _ _ = False

{-
These all now work
eq [1::Int,2::Int,3::Int] [1::Int,2::Int,3::Int]
eq (1::Int) (2::Int)
eq (1::Int) (2::Int)
eq ((1::Int):(2::Int):[]) ((1::Int):(2::Int):[])
eq ((1::Int):(2::Int):[]) ((2::Int):(2::Int):[])
eq ([]::[Int]) ([])
eq ([]::[Test]) ([])
eq ([]::[Char]) ([])

-}
On 01/12/13, Brent Yorgey <byorgey@seas.upenn.edu> wrote:

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
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://www.haskell.org/mailman/listinfo/beginners