$ cat maybe.hs handleMaybeNum :: Maybe int -> int handleMaybeNum Just a = a $ ghci maybe.hs GHCi, version 7.10.3: http://www.haskell.org/ghc/

$ cat maybe.hs handleMaybeNum :: Maybe int -> int handleMaybeNum Just a = a $ ghci maybe.hs GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( maybe.hs, interpreted ) maybe.hs:2:16: Constructor ‘Just’ should have 1 argument, but has been given none In the pattern: Just In an equation for ‘handleMaybeNum’: handleMaybeNum Just a = a Failed, modules loaded: none. You're right about the "precompile time error", although it's not what you claim... Let's fix the syntax error: $ cat maybe_fixed.hs handleMaybeNum :: Maybe int -> int handleMaybeNum (Just a) = a $ ghci maybe_fixed.hs GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( maybe_fixed.hs, interpreted ) Ok, modules loaded: Main. *Main> No error now. You do get a warning with -Wall, but that's not quite what you claimed. (Newer versions may be more strict, I don't know.)