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