CODEWITHSUNDEEP
haskell
Srinivas
Srinivas

Reputation: 2098

How do i extend Haskell's type class in my class

I want to extend Show type class within my type class and then provide implementation for both of the classes

data XX = XX String --class

class Show a => Test a where
    mySh :: a -> String
    show :: a -> String

instance Test XX where --implementation for both show and mySh
    mySh (XX a) = a
    show (XX a) = "---" ++ a

I am getting this error

    • No instance for (Show XX)
    arising from the superclasses of an instance declaration
• In the instance declaration for ‘Test XX’

I just want Test to inherit Show (without the default implementation) and provide it's own methods.

Upvotes: 1

Views: 183

Answers (1)

leftaroundabout
leftaroundabout

Reputation: 120751

The subclass relation Show a => Test a (read: Show is a superclass of Test) simply means that any type which is an instance of Test must also be an instance of Show, i.e. pretty much exactly what GHC is telling you. To this end, well, write that instance:

instance Show XX where
  show (XX a) = "---" ++ a  -- please don't implement it this way, `show`
                            -- is supposed to yield valid Haskell code
instance Test XX where
  mySh (XX a) = a

In practice, the Show class is almost always best instantiated with a simple deriving statement:

data XX = XX String
  deriving (Show)

instance Test XX where
    mySh (XX a) = a

What you can not do in Haskell, is “override” the show method. That's something OO languages do to get a different run-time behaviour on a subtype, which is basically what an OO child class is. Subtype because it contains values that can also be used as values of the parent class. But Haskell does not have subtypes. You may rather think of classes as sets of types, and a subclass of the Show class is a subset of the set of types which have a Show instance.

Upvotes: 4

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