gistfile1.txt

Warning: The package list for 'hackage.haskell.org' is 18 days old.
Run 'cabal update' to get the latest list of available packages.
Resolving dependencies...
Configuring lisp-0.1.0.0...
Building lisp-0.1.0.0...
Preprocessing executable 'lisp' for lisp-0.1.0.0...
[1 of 1] Compiling Main             ( Main.hs, dist/build/lisp/lisp-tmp/Main.o )

Main.hs:20:6:
    Unexpected semi-colons in conditional:
        if null sign then (return $ Value . Number $ read num); else return
                                                                     $ Value
                                                                     . Number
                                                                     . negate
                                                                     $ read num
    Perhaps you meant to use -XDoAndIfThenElse?
cabal: Error: some packages failed to install:
lisp-0.1.0.0 failed during the building phase. The exception was:
ExitFailure 1