New instance declaration for Show

Question

I\'m trying to add an instance declaration in Haskell for a new data type I\'ve created unsuccessfully. Here what I\'ve tried so far:

data Prediction = Predictio         


        

Answer 1

You've got the syntax for instances wrong. To create an instance of Show write:

instance Show Foo where
  show = ...
  -- or
  show x = ...

where ... contains your definition of the show function for Foo.

So in this case you want:

instance Show Prediction where
  show = showPrediction

or, since there isn't an important reason to have showPrediction at all:

instance Show Prediction where
  show (Prediction a b c) = show a ++ "-" ++ show b ++ "-" ++ show c