I finally find the bug: there should have a key word 'where' at the end of first line, like the following code:

module FormatDuration where

formatDuration :: (Integral i) => i -> String
formatDuration n = "now"

However, the default code (click the 'RESET' button to get it) doesn't have it.
Please someone fix it.

Always get parse error.

/tmp/haskell1171111-5-1hh3nhb.1k7m5z5mi/FormatDuration.hs:3:1:
parse error on input `formatDuration'

I got this error with the following code:

module FormatDuration

formatDuration :: (Integral i) => i -> String
formatDuration n = "now"