data KeyValuePair a b = KeyValuePair String String
'a' and 'b' are called phantom types and they are used to tag a datatype. Rather than explain what that means in some abstract way I'll just show you how I have used them: I had the following simple program that retrieved values from and wrote to a dictionary data structure (like Oxford's or Webster's). Here are the types in Haskell: type Name = String type Definition = String type Dictionary = [(Name,Definition)] The actual contents of the dictionary were stored in a SQLite database so I needed to read them into the "Dictionary" type whenever I started up the program. And I had a number of functions that queried the "Dictionary" that required on a populated dictionary and I wanted to check *at compile time* that they would only be called on a populated dictionary. So I added the following two datatypes: data Populated = Populated data New = New I then changed my "Dictionary" to take an extra argument that is never used just like your "KeyValuePair": type Dictionary a = [(Name,Definition)] Now I could write a function that populated the dictionary: populateFromDatabase :: Dictionary New -> IO (Dictionary Populated) and query functions: getDefinition :: Name -> Dictionary Populated -> Definition Since "populateFromDatabase" is the only function that can create a "Dictionary Populated" the phantom type statically guarantees that "getDefinition" and any other function that expects a full dictionary can never be called before "populateFromDatabase". The Haskell Wiki has a similar example [1] and Richard Jones has a nice article on they are used in Ocaml [2] . hth, -deech [1] http://www.haskell.org/haskellwiki/Phantom_type [2] http://camltastic.blogspot.com/2008/05/phantom-types.html