Currently CircleCI automatically detects the presence of a *.cabal file for running tests, but it does not appear to detect stack.yaml, which indicates that the project uses Haskell Stack, a more recent build tool for Haskell projects.
Stack is gaining more ground due a nicer configuration, reproduceable builds, and automatic support for NixOs, Docker, and sandboxes. Supporting it out of the box would be nice.
Please note that Stack projects will still have a *.cabal file available. The presence of both indicates that the project should use stack and not cabal.