Re: [Haskell-cafe] bug in ghci ?

On Thursday 08 July 2010 18:24:05, Ben Millwood wrote:

On Thu, Jul 8, 2010 at 3:45 PM, Daniel Fischer wrote:

...

Well, I made the suggestion of emitting a warning on instance declarations without method definitions. That would be comparatively easy to implement (even with an additional check to only emit the warning if the class defines any methods) and catch many (if not most) cases.

Unfortunately, it would catch some perfectly valid cases, see the list of instances for flat datatypes here:

http://hackage.haskell.org/packages/archive/deepseq/1.1.0.0/doc/html/src /Control-DeepSeq.html

This demonstrates that there is at least one (admittedly probably not much more than one) case where a class with methods would have a default implementation that was total and valid in some cases.

Good point. So one should check for more than one class-method [then defining no methods in the instance declaration is likely to lead to a default-method loop if there are default methods for all, otherwise GHC will warn already]. That can of course still give rise to spurious warnings, but is less likely to.