Here is what I was able to come up with: module Store where data FStore1 a b = Empty1 | FS1 ( a->b , FStore1 a b ) store1 :: (a->b) -> (FStore1 a b) store1 f = let x = FS1 (f, x) in x data FStore2 a b = Empty2 | FS2 ( a->b , FStore2 a b, FStore2 a b ) store2 :: (a->b) -> (FStore2 a b) store2 f = let x = FS2 (f,x,x) in x First I tryed store1, but after a while it went out of business. Second I tryed other business model with store2. Same result. Then I tryed to make a street of stores. After a while, they all went out of business. Then I tryed to make several streets of stores. After a while they all went out of business. Then I tryed to make an infinite street of stores. After a while they all went out of business. Then, following some rumors that there is a store on that street with some function in stock that might be willing to give it away, I noticed that the street disappeared, and I cannot go to that store. Can you help me find it ?