Tom Ellis wrote:
On Mon, Aug 15, 2016 at 03:40:46PM +0100, Malcolm Wallace wrote:
It is no surprise that the instance Eq A is not mentioned, because there is a perfectly valid instance for it already, and it contains no error.
I disagree.
There is no valid instance for Eq (A B), because all that the compiler knows is that 'Eq B => Eq (A B)', and there is no 'Eq B'.
There are two separate issues here.
1. The reason the compilation fails is that there is no instance 'Eq (A B)'.
2. The reason there is no instance 'Eq (A B)' is that there is no instance 'Eq B'.
I believe the the missing Eq instance for B is the more useful bit of information of these two, but I wouldn't mind an explanation of the simplification steps that the compiler made to discover it. I'm a bit worried though that such an explanation may be very long, and I expect that it's not trivial to extract this information from ghc's constraint based type checker. (In principle, -ddump-tc-trace prints all this information, but it's far too verbose and cluttered by ghc internal identifiers.) Cheers, Bertram