The haskell-platform package is not actually listed on Hackage. Consequently, the homepage URL auto-generated by cabal2arch is incorrect.
Maybe it would be better to use the homepage property from the CABAL file, if it exists?
In case of haskell-platform, that is definitely a good choice. I'm not so sure about other packages. The homepage URL on hackage.haskell.org is going to be stable until the end of time. The same can probably not be said about the URLs that people write into their cabal files. Anyway, if the author of the package says that his homepage is at XYZ, then it can't be wrong for us to link to XYZ. All things considered, I'm in favor of making that change.