On Dec 30, 2007, at 8:24 AM, Joost Behrends wrote:
For adapting hws (one of the reasons for me to be here, not many languages have a native web server) to Windows i must work on time. In System.Time i found
data ClockTime = TOD Integer Integer
2 questions arise here: Does this define "TOD" (which i do not find elsewhere) together with ClockTime also ? And: Why is this not:
data ClockTime Integer Integer = TOD Integer Integer ?
Is it just an abbreviation for the first? Or is there a connection to ClockTime as an "abstract data type" (a notion, which would have a subtle different meaning than in OOP - since "instance" is such different thing here).
You are right that it defines the TOD constructor. As for you second question, I will try to be somewhat formal in my response, so hopefully I don't just throw you off more. The quick answer is that since we already know the parameters on the right side are Integers, we don't need to specify them on the left side. When you define datatypes, you are essentially defining a type-level constructors on the left hand side and (value-level) constructors on the right hand side. Just like normal functions, constructors and type constructors can be parameterized. Let's deviate for a moment from Haskell's notation for data types and approach this from the viewpoint of a dependently typed language (a language in which there is little separating between the type level and the value level). The data type we are defining here is called ClockTime, so its type might be represented as ClockTime :: * , where * represents "Kind," the type of types. For completeness, the sole constructor we define is called TOD and has type TOD :: Integer -> Integer -> ClockTime . 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. This (1) does not make much sense in a language like Haskell which doesn't have true dependent types, and (2) does not help us in any way with our definition of the TOD constructor. We already know by the definition of TOD that it takes two Integers and returns a ClockTime. If we used this modified definition of ClockTime, we would have to parameterize it to specify TOD, maybe like. TOD' :: Integer -> Integer -> ClockTime' 2 3 (I chose the 2 and 3 arbitrarily, but these exact values have no particular relevance here.) This would not work in Haskell. However, there are cases where you would want to parameterize a type constructor. For example, say we _wanted_ our TOD constructor take two values of arbitrary types. If we want the type level to reflect the types of these parameters, ClockTime must be parameterized. ClockTime'' :: * -> * -> * TOD'' :: a -> b -> ClockTime'' a b In Haskell notation, this would be equivalent to data ClockTime'' a b = TOD'' a b . So, to summarize, the reason that we don't use data ClockTime Integer Integer = TOD Integer Integer is because we don't want to parameterize ClockTime, and even if we did, we could not use Integer values like this to do it because Haskell is not dependently typed. - Jake