PaaS (platform as a service) cloud providers generally limit you to a fixed technology stack. However, OpenShift and Heroku allow third-party extensions, which can be used to support Haskell.
The cartridge comes in several flavours, with just the network package (GHC 7.8.2) or a pre-installed framework (GHC 7.6.3 *):
* GHC 7.8 requires dynamic libraries for Template Haskell and Quasiquoting. OpenShift gears default to a 1Gb quota, which is insufficient for a full web framework with two copies (dynamic and static) of every library.