I’ve always found it difficult to remember the exact syntax for setting Ghci into multiline mode and defining a function therein. Below are the steps for easy access.

Start by setting Ghci into multiline mode with:

:set +m

You can start a multiline block with :{ and end it with :}.

Function definitions must be preceded with let. This has tripped me up many times.

For example, to define a function that pauses for a given delay before printing out “done”:

  let printAfter :: Int -> IO ()
      printAfter delay =
        do putStrLn $ (\d -> "waiting for " ++ d ++ " microseconds") $ show delay
           threadDelay delay
           putStrLn "done"

To unset multiline mode use:

:unset +m

References: Multi-line commands in GHCi, How to define a function in ghci across multiple lines?