Generate triple (Network.HTTP.ResponseCode) using an instance of Arbitrary

Question

I have a function that takes a ResponseCode from Network.HTTP. In order to test it with QuickCheck, I wanted to write an instance of Arbitrary for ResponseCode. (In case you don't know, a ResponseCode is just a triple of ints in that library: type ResponseCode = (Int, Int, Int)).

So I wrote something like this:

instance Arbitrary ResponseCode where
    arbitrary = triple ( elements [1..6] )
       where triple f = (f, f, f)

First of all, GHC complains that the way I'm using types is not standard haskell so I would have to use some compiler flags (which is not really what I want since I feel like there must be an easy solution for this simple problem without flags).

Secondly, my arbitrary function has the wrong type, which is pretty obvious. But then I really didn't figure out how to write a function that returns a triple with random Ints ranging from 1-6.

I would appreciate if somebody could help me out here.

Thank you.

Answer 1

First of all, there is already are these two instances:

instance Arbitrary Int
instance (Arbitrary a, Arbitrary b, Arbitrary c) =>
         Arbitrary (a, b, c)

This means that (Int,Int,Int) is already an instance of Arbitrary. This means that the type synonym ResponseCode is already an instance. You cannot define and use a second instance.

You could try using Test.QuickCheck.Gen.suchThat but I hypothesize that it will not work well in this case. If you can, I suggest using a newtype wrapper:

import Test.QuickCheck
import Network.HTTP.Base
import Control.Applicative
import System.Random

newtype Arb'ResponseCode = Arb'ResponseCode { arb'ResponseCode :: ResponseCode }
  deriving (Show)

instance Arbitrary Arb'ResponseCode where
  arbitrary = do
      responseCode <- (,,) <$> f <*> f <*> f
      return (Arb'ResponseCode responseCode)
    where f = elements [1..6]

-- To use this you can call
-- (fmap arb'ResponseCode arbitrary)
-- instead of just (arbitrary)

Or perhaps use the built-in (Int,Int,Int) instance and post-process the three elements with (succ . (mod 6)).