Re: [Haskell-cafe] Declaring a tuple of instances of Enums as an instance of the Enum class

R J writes:

Say I've got a type "Month" declared as an instance of the "Enum" class, and a type "MonthPair" declared as a pair of months: data Month = January | February | March | April | May | June | July | August | September | October | November | December deriving (Eq, Enum, Ord, Show) type MonthPair = (Month, Month) deriving (Enum) The "deriving" on "MonthPair" gives me the error "parse error on input deriving'".

You can't derive instances for type aliases (as its mainly there for documentation reasons, etc.). However, pairs don't have Enum instances by default so you still can't use its instance. If you define "data MonthPair = MonthPair Month Month" then you should be able to derive Enum. -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com IvanMiljenovic.wordpress.com