Haskell: Non-exhaustive patterns in function checkDirecRio-CodePudding

The next file complile but when I call the checkDirecRio it gives me :Non-exhaustive patterns in function checkDirecRio


checkDirecRio :: Mapa -> Bool
checkDirecRio (Mapa _ []) = True
checkDirecRio (Mapa larg ((Rio x, _) : (_,_)  : t)) = checkDirecRio (Mapa larg t)
checkDirecRio (Mapa larg ((Rio x, _) : (Rio y, _) : t)) | ((x > 0 && y < 0) || (x < 0 && y > 0)) = True
                                                        | otherwise = checkDirecRio (Mapa larg t)

which the structure Mapa is defined as:


type Velocidade = Int

data Mapa =
  Mapa Largura [(Terreno, [Obstaculo])]
  deriving (Show, Read, Eq)


data Terreno
  = Rio Velocidade
  | Estrada Velocidade
  | Relva
  deriving (Show, Read, Eq)


data Obstaculo
  = Nenhum -- ^ nothing
  | Tronco -- ^ wood who can only in Rio being
  | Carro -- ^ car who can only in Estrada being
  | Arvore -- ^ tree who can only in Relva being
  deriving (Show, Read, Eq)

I expect that the code can be able to validate if the river Rio has different direction, return False if 2 contiguous rivers has the same direction as True if contiguous rives has opposite directions

CodePudding user response：

The function checkDirecRio has incomplete patterns because it does not cover all the variants of Terreno which has 2 more variants besides Rio. You should include the case that covers them as well. For example:

checkDirecRio (Mapa _ (((Estrada _), _):_)) = True
checkDirecRio (Mapa _ ((Relva, _):_)) = False
checkDirecRio (Mapa _ [((Rio _), _)]) = False