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.
DigitalOcean gives you a clean server that you can SSH to. Supports Docker as well.
The cartridge comes in several flavours, with just the network and text packages or a pre-installed framework:
cabal update is likely to exceed the memory limit on small (free) gears due to cabal issue #2396.
Heroku buildpack for tryhplay, incorporating Haste and HPlayground. Demo at http://tryplayg.herokuapp.com/.