The type checker can provide rapid feedback as you code. We recommend using ghcid. It is a small application that runs in your terminal, watches for changes in your source files, and displays any errors that the compiler detects.
ghcid with this command.
$ ghcid --command 'ghci hello-world.hs'
Since our program produces no warnings or errors, the display says “All good”.
All good (1 module, at 00:34:33)