On Donnerstag, 6. Dezember 2012, 12:51:55, Trung Quang Nguyen wrote:
Hi there,
After installing c2hs, and run cabal install bullet. I got this error. Did I miss any step?
No, the maintainer of bullet just has not updated the package to incorporate the changes in base-4.6.
Physics/Bullet/Raw/C2HS.hs:211:12: Could not deduce (Eq a) arising from a use of `toBool' from the context (Num a) bound by the type signature for cToBool :: Num a => a -> Bool at Physics/Bullet/Raw/C2HS.hs:211:1-17 Possible fix: add (Eq a) to the context of the type signature for cToBool :: Num a => a -> Bool In the expression: toBool In an equation for `cToBool': cToBool = toBool
Formerly, Eq and Show were superclasses of Num ( class (Eq a, Show a) => Num a where... ) but they have been removed recently (they prevented some reasonable Num instances unless you mad dummy Eq and Show instances, instance Num a => Num (e -> a) for example). So before base-4.6, a Num constraint implied Eq, that is no longer so. Quick fix: $ cabal unpack bullet - cd bullet-0.2.2 - edit the .cabal file, bumping the version to 0.2.2.1 or so - edit the sources adding Eq or Show constraints where necessary (you have seen one spot, cabal configure and after that cabal build would detect further spots if there are any, cabal install too, but if you configure with --disable-library-profiling --disable-shared, cabal build is quicker to find the spots) - cabal install (no arguments, so it configures and installs the package from the directory) Long-term fix: send a patch (or at least a bug report/feature request) to the maintainer of bullet.