[Haskell-cafe] bug in ghci ?

Daniel Fischer daniel.is.fischer at web.de
Thu Jul 8 12:48:34 EDT 2010

On Thursday 08 July 2010 18:24:05, Ben Millwood wrote:
> On Thu, Jul 8, 2010 at 3:45 PM, Daniel Fischer <daniel.is.fischer at web.de> 
> > 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/
> 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 
That can of course still give rise to spurious warnings, but is less likely 

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