How do I write, “if typeclass a, then a is also an instance of b by this definition.”

Question

I have a typeclass MyClass, and there is a function in it which produces a String. I want to use this to imply an instance of Show, s

Answer 1

As Ed Kmett pointed out, this is not possible at all for your case. If however you have access to the class you want to provide a default instance for, you can reduce the boilerplate to a minimum with a default implementation and constrain the input type with the default signature you need:

{-# LANGUAGE DefaultSignatures #-}

class MyClass a where
    someFunc :: a -> Int

class MyShow a where
    myShow :: a -> String
    default myShow :: MyClass a => a -> String
    myShow = show . someFunc

instance MyClass Int where
    someFunc i = i

instance MyShow Int

main = putStrLn (myShow 5)

Note that the only real boilerplate (well, apart from the whole example) reduced to instance MyShow Int.

See aesons ToJSON for a more realistic example.