CODEWITHSUNDEEP
haskell
Paul Kar.
Paul Kar.

Reputation: 1324

Lazy Patterns in Haskell

Could someone explain to me what this code snippet does (mainly last two lines)? Shouldn't I be able to pass Nothing to defValue? I tried and got an error "No instance for (Def, a0) arising from a use of 'defValue'"

class Def a where 
  defValue :: a -> a

instance Def a => Def (Maybe a) where
  defValue ~(Just x) = Just (defValue x)

Upvotes: 1

Views: 139

Answers (1)

leftaroundabout
leftaroundabout

Reputation: 120731

This problem has nothing to do with patterns, in fact you would have got the same error message if you'd declared

instance Def a => Def (Maybe a) where
  defValue = undefined

In fact, your code should work (as far as it possibly can, however I'd say your entire idea isn't good, very unsafe without need), but to use that Maybe instance you need an instance of the packed type to work with. For instance, if you add

instance Def Int where
  defValue _ = 0

then you can do

DefVals> defValue (Just 1 :: Maybe Int)
Just 0
DefVals> defValue (Nothing :: Maybe Int)
Just 0

Upvotes: 3

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