This is definitely weaker than a SLIME-like REPL, but I use ghcid --warnings --test My.Module.Tests.runTest, where runTest is tasty's defaultMain, to get near-instantaneous test running on changes to files. (Since it's GHCi instead of GHC, waiting for a full compile is unnecessary, and it still has the necessary smarts about what needs reloading to avoid reloading the whole project every change.)