Syntax error: parse error on input ' -> ' )

Question

I have a problem in this code:

module Blueprint where

data Colour = Blue | Green | Red 
  deriving ( Eq, Show )

data Car = Car { wheels :: Integer
               , colour :: Colour
               }
  deriving ( Eq, Show )

data Property = Colour_Is Colour
              | Wheels Ordering Integer
              | And Property Property
              | Not Property
              | Or Property Property 
  deriving Show 

check :: Property -> Car -> Bool
check prop car = case prop of
    Colour_Is col -> col == colour car
    Wheels ord num -> case ord of 
                            LT -> num > wheels car
                            EQ -> num == wheels car
    And l r -> check l car && check r car
    Not p -> not check p car
    Or x y -> check x car || check y car 


cars = [ Car { wheels = 4, colour = Red }
        , Car { wheels = 2, colour = Blue }
        , Car { wheels = 14, colour = Green }
        , Car { wheels = 4, colour = Green }
        , Car { wheels = 2, colour = Red }
        ]

prop1 :: Property
prop1 =  And (Wheels EQ 14) (Colour_Is Green)

test :: Bool
test = and
    [ check ( Wheels EQ 4 ) ( cars !! 0 )
    , check ( Wheels LT 3 ) ( cars !! 1 )
    , check ( And ( Wheels EQ 14 ) ( Colour_Is Green )) ( cars !! 2 )
    , check ( Not ( Colour_Is Red ) ) ( cars !! 3 )
    , filter ( check prop1 ) cars == take 3 cars
    ]

I built the check function and implemented Colour_Is,And,Not and Wheels correctly, but when I add the Or function:

( Or x y -> check x car || check y car )

I'm getting this error from ghci :

D:\My_data\hs\strategy.hs:26:16: parse error on input `->' Failed, modules loaded: none.

I'm new to Haskell. Where is my mistake?

Answer 1

Your parse error is probably due to incorrect indentation, but that error doesn't exist with Dhaivat's reformatting of your question.

Further, you needed parens around the argument for not:

  Not p -> not check p car
-->
  Not p -> not (check p car)

In addition to properly formatting your question, next time please pick a sensible title. The title of "Haskell doesn't work" is a) wrong b) uninformative c) not conductive to getting an answer from people who like Haskell. It's still a small enough community that may people with a personal investment in the Language (people who've worked on the language spec and core libraries) are actually part of the language's social scene.