ghc --make expre.hs -o expre
Chasing modules from: expre.hs
Compiling Main             ( expre.hs, expre.o )

expre.hs:29:
    Couldn't match `Parser Integer' against `t -> t1'
        Expected type: Parser Integer
        Inferred type: t -> t1
    Probable cause: `expr' is applied to too many arguments in the call
        (expr "1+2*3")
    In the definition of `main': main = expr "1+2*3"