Add property tests to resistor-color-duo

[tejasbubane]

Closes #867

[sshine]

Hi @tejasbubane, and thanks for your PR!

I'll get back to it in the next few days unless someone else does. :-)

[sshine]

Nice work so far.

I've left some comments below for improvement.

For interesting reading material,

https://www.fpcomplete.com/blog/quickcheck-hedgehog-validity

It may also be worth linking to in the link collection in your hint.

[sshine]

Three things:

• Avoid orphan instances by using a newtype rather than -fno-warn-orphans.
• Exhaust the data type programmatically by deriving (..., Enum, Bounded).
• Use elements :: [a] -> Gen a instead of oneof here.

Suggested change

	instance Arbitrary Color where
	arbitrary = oneof $ map return [Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, White]
	newtype AnyColor = AnyColor Color deriving (Show, Eq)
	instance Arbitrary AnyColor where
	arbitrary = AnyColor <$> elements [minBound..]

But instead of using the Arbitrary type class, consider simply making a colorGen:

Suggested change

	instance Arbitrary Color where
	arbitrary = oneof $ map return [Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, White]
	colorGen :: Gen Color
	colorGen = elements [minBound..]

I might also place this generator at the bottom of the file.

[sshine]

See comment below above.

Suggested change

{-# OPTIONS_GHC -fno-warn-orphans #-}

[sshine]

I haven't seen that we use "Bonus" or mark sub-subsections using **...** elsewhere on the track.

I might skip this heading altogether.

[sshine]

Hey @tejasbubane, are you interested in following up on this PR?