[Haskell-beginners] Install bullet-0.2.2 failure in OS X 10.8.2

[Trung Quang Nguyen]

Thanks a lot Daniel. I will try to find out. I have just installed Haskell
today, so I don't understand much about these error messages.

Best regards,
Trung


2012/12/6 Daniel Fischer <daniel.is.fischer at googlemail.com>

> 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.
>



-- 
*Trung Nguyen*
Mobile: +45 50 11 10 63
LinkedIn: http://www.linkedin.com/pub/trung-nguyen/36/a44/187
View my blog at http://www.onextrabit.com/
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