Haskell is officially supported on Scalingo
It uses the cabal packaging system.
Usage
$ ls
Procfile app.cabal src
$ scalingo create my-app
$ git push scalingo master
...
-----> Haskell app detected
-----> Building app with cabal
-----> Running: cabal install
...
Buildpack
More information at https://github.com/Scalingo/haskell-buildpack.