Hello, List! For example, I have specialized (right nameis phantom?) type: data Day a = Day { ... no `a` here } data Sunny data Rainy joyToday :: Day Sunny -> IO () joyToday day = ... melancholyToday :: Day Rainy -> IO () melancholyToday day = ... And I can create (in spite of that it's phantom) some day: let day1 = Day {...} :: Day Sunny joyToday day1 but no problem to create `Day Int`, `Day Char`, etc which is pointless actually (sure "creator"-function can be exported from the module only, but I'm talking about type-level solution). I know that constraints (`... =>`) on data types are redundant/removed from the language. And I'm not sure how it's possible to restrict that parameter `a` (I know that it's possible to Java/C++/Perl6 (not sure), some other languages but how to add such restriction in Haskell? IMHO type families can help but I'm not sure how it will look (Sunny, Rainy are "nullary" type, so...). Is it possible for Haskell too? === Best regards, Paul