Hi,
On 25 August 2015 at 14:18, Johan Tibell
The proposed change to my library is here: https://github.com/tibbe/cassava/pull/95/files
We remove the OverlappingInstances pragma and instead add an OVERLAPPABLE pragma like so:
instance {-# OVERLAPPABLE #-} FromField a => FromField (Maybe a) where
This causes clients of the library that previously compiled (e.g. the music-parts package) to no longer compile, due to a now lacking OVERLAPPING pragma in their code.
No, it's not quite like that. Client code can start to break when {-# LANGUAGE OverlappingInstances #-} is removed, as happened with the music-parts package. Adding an OVERLAPPABLE pragma to cassava's code made that error go away. Client code can usually work around the problem of missing OVERLAPPABLE pragmas in the library by adding OVERLAPPING pragmas to their instances. The reason I suggested bumping cassava's version is that there may be some places in cassava that still need new pragmas that I've overlooked. If GHC had an option for detecting overlapping instances at definition site, that'd help, I think, since then it'd be easier to find instances that need new pragmas.