fork download

copy

data DataOrError a b = Error a | Data b deriving Show
 
apply f (Data x) (Data y) = Data (f x y)
apply f (Data x) y@(Error _) = y
apply f x@(Error _) _ = x
 
main = do
  print (apply (+) x1 x2) -- (1)
  print (apply (+) x1 e2) -- (2)
  print (apply (+) e1 x2) -- (3)
  print (apply (+) e1 e2) -- (4)
  where
    x1 = Data (2 :: Int)
    x2 = Data (3 :: Int)
    e1 = Error ("First thing failed")
    e2 = Error ("Second thing failed")
 

[1 of 1] Compiling Main             ( prog.hs, prog.o )

prog.hs:8:3:
    No instance for (Show a0) arising from a use of `print'
    The type variable `a0' is ambiguous
    Possible fix: add a type signature that fixes these type variable(s)
    Note: there are several potential instances:
      instance (Show a, Show b) => Show (DataOrError a b)
        -- Defined at prog.hs:1:50
      instance Show Double -- Defined in `GHC.Float'
      instance Show Float -- Defined in `GHC.Float'
      ...plus 24 others
    In a stmt of a 'do' block: print (apply (+) x1 x2)
    In the expression:
      do { print (apply (+) x1 x2);
           print (apply (+) x1 e2);
           print (apply (+) e1 x2);
           print (apply (+) e1 e2) }
    In an equation for `main':
        main
          = do { print (apply (+) x1 x2);
                 print (apply (+) x1 e2);
                 print (apply (+) e1 x2);
                 .... }
          where
              x1 = Data (2 :: Int)
              x2 = Data (3 :: Int)
              e1 = Error ("First thing failed")
              e2 = Error ("Second thing failed")

Standard output is empty