Own perspective: You need fpc and to build that you need fpc - again as binary-package. So you have to work up the whole stack from the ground. We have integrated fpc not for now!

You need a version of fpc to compile fpc and therefore create the compiler. Afterwards you have a working package, you can create ghc and from that step by step all the rest of Haskell. It is a problematic way because you can download the binary package for fpc which is therefore resulting to be done for compilation the first package - for updates it would be done then with the new compiled package in the future going. So also the reasoning until now not integrating that!

Especially those packages you have mentioned in the first post are the reasoning we have no haskell for now, because the integration is not possible without binary releases and that is a real problem. Parabola GNU/Linux is importing packages from Arch Linux, so they have a starting point. We have this not and it would violate the concept for upcoming releases also. When you start from scratch you will be confronted with the essential problem of fpc in some comparable way: Your PKGBUILD would have to contain a binary-release of it to compile it.