2007/12/30, Joost Behrends <[EMAIL PROTECTED]>: > > . Now, let's say we had tried defining ClockTime with parameters as > > you suggested. > > > > ClockTime' :: Integer -> Integer -> * > > > > Do you see the problem? In order to use the ClockTime type > > constructor, we would have to use Integer values. > > Cannot see any problem here - do we NOT want ClockTime to be initialized by > two > Integers ? Or is this the main reason for introducing "TOD" - to be able to > change it without having to make any changes to code using ClockTime ? > To repeat myself - am i right understanding, that this needs a differently > named > data constuctor ?
We want a value of type ClockTime to be initialized by two Integers, but the type of this value will be ClockTime, not (ClockTime 2 3), its _type_ won't depend on its _content_, ok ? With dependent types, you can have type constructors (not data constructors) with an non-type parameter, but Haskell hasn't dependent typing. To give you an example of the potential usefulness of dependent type, you can have a list type which is parametrized by the length of the list. data List :: * -> Integer -> * where Empty :: List a 0 Cons :: a -> List a n -> List a (n+1) you then can type head() so that you can't apply it on an empty list : head :: List a n -> List a (n-1) | n > 0 head (Cons x xs) = xs and the compiler will enforce this restriction at compile time (as well as it can). Dependent typing is interesting but not really practical yet. You can simulate it more or less with Haskell and GHC extensions, but Haskell isn't dependently typed. -- Jedaï _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
