Mystery program

What does this do?

	#!/usr/bin/env stack
	-- stack --resolver=lts-12.6 script

	module MysteryProgram where

	import Data.List (unfoldr)

	class Eq a => Gen a where
	next :: a -> a

	gen :: Gen a => a -> a -> [a]
	gen from to =
	unfoldr
	(\b -> if b == to then Nothing else Just (b, next b))
	from

	mystery :: Gen a => [a] -> [a]
	mystery xs@(x : _) = go x xs
	where
	go curr (x : xs)
	\| x == curr = go (next x) xs
	\| otherwise = gen curr x ++ go (next x) xs
	go _ _ = []
	mystery [] = []

	instance Gen Int where
	next = (+) 1

	main :: IO ()
	main = do
	print $ mystery ([2, 14, 15, 16, 19] :: [Int])
	print $ mystery ([2, 6] :: [Int])
	print $ mystery ([2, 5] :: [Int])
	print $ mystery ([2, 4] :: [Int])
	print $ mystery ([2, 3] :: [Int])
	print $ mystery ([2] :: [Int])
	print $ mystery ([] :: [Int])

view raw MysteryProgram.hs hosted with ❤ by GitHub

There are no cash prizes.

Update: 2019-01-26

My friend Dave Compton suggested the following simplification:

	#!/usr/bin/env stack
	-- stack --resolver=lts-12.6 script

	import Data.List (unfoldr)

	class Eq a => Gen a where
	next :: a -> a

	gen :: Gen a => a -> a -> [a]
	gen from to =
	unfoldr
	(\b -> if b == to then Nothing else Just (b, next b))
	from

	mystery :: Gen a => [a] -> [a]
	mystery (x : xr) = go (next x) xr
	where
	go curr (y : ys)
	= gen curr y ++ go (next y) ys
	go _ _ = []
	mystery [] = []

	instance Gen Int where
	next = (+) 1

	main :: IO ()
	main = do
	print $ mystery ([2, 14, 15, 16, 19] :: [Int])
	print $ mystery ([2, 6] :: [Int])
	print $ mystery ([2, 5] :: [Int])
	print $ mystery ([2, 4] :: [Int])
	print $ mystery ([2, 3] :: [Int])
	print $ mystery ([2] :: [Int])
	print $ mystery ([] :: [Int])

view raw MysteryProgram2.hs hosted with ❤ by GitHub

Thanks!

blog.rcook.org