Re: [Haskell-cafe] Typeclass resolution errors quite puzzling

On Mon, Aug 15, 2016 at 03:33:23PM +0200, Damian Nadales wrote:

On Mon, Aug 15, 2016 at 3:26 PM, Tom Ellis wrote:

...

Here's a program with an odd error message (GHC 8.0.1):

data A a = A a deriving Eq data B = B

main :: IO () main = print (A B == A B)

test/main.hs:5:15: error: • No instance for (Eq B) arising from a use of ‘==’ • In the first argument of ‘print’, namely ‘(A B == A B)’ In the expression: print (A B == A B) In an equation for ‘main’: main = print (A B == A B)

I would expect this error, since B does not implement equality. Maybe you're expecting Haskell to automatically derive an Eq B instance, but that is not the behavior I would want, since data definitions could implicitly define instances for other data types.

Is there any reason why you don't define: data B = B deriving Eq?

My complaint is about the confusing error message, not the failure to type check.