Declare all instances of a typeclass are in another typeclass without modifying the original class declarations

Question

There is an Crypto.Random API inside the crypto-api package that specifies what it means for something to be a \"pseudorandom number generator\".

I have implemented this

Answer 1

As far as I know, this is impossible, unless you're willing to turn on UndecidableInstances (which, of course, can make the typechecker go in an infinite loop). Here's an example that makes every instance of Monad an instance of Functor:

{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}

module Main
       where

import Control.Monad (liftM)

instance (Monad a) => Functor a where
    fmap = liftM


-- Test code
data MyState a = MyState { unM :: a }
               deriving Show

instance Monad MyState where
  return a = MyState a
  (>>=) m k = k (unM m)

main :: IO ()
main = print . fmap (+ 1) . MyState $ 1

Testing:

*Main> :main
MyState { unM = 2 }

In your case, this translates to:

{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}

instance (RandomGen a) => CryptoRandomGen a where
  newGen = ...
  genSeedLength = ...
  genBytes = ...
  reseed = ...

As an aside, I once asked how to implement this without UndecidableInstances on haskell-cafe and got this answer (the same workaround that Thomas proposed; I consider it ugly).