GHCid

ghcid is a minimalist development tool for Haskell that runs GHCi as a daemon, watches source files for changes, reloads automatically, and shows compile errors instantly—providing a tight edit-feedback loop. In general, to use ghcid, you first need to get ghci working well for you. In particular, craft a command line or .ghci file such that when you start ghci it has loaded all the files you care about (check :show modules). If you want to use --test check that whatever expression you want to use works in that ghci session. Getting ghci started properly is one of the hardest things of using ghcid, and while ghcid has a lot of defaults for common cases, it doesn't always work out of the box. Expressions that read from standard input are likely to hang, given that Ghcid already uses the standard input to interact with Ghci.