The overall goal here is to make sure that when you have a function that takes a Date or Time type, the values are consistent with the intended invariants of the type! On Wed, 2 Feb 2005, Marcin 'Qrczak' Kowalczyk wrote:
or 4 digits. Why not define year like this:
newtype Year = Year Int -- don't export constructor mkYear century centyear = Year (100*century+centyear) mkBigYear millenium cent centyear = ...
Because it's easier to use the actual year number.
Ok, but we have the common enough case of 2 digit vs 4 digit year that we might as well handle it. The Y2K problem is an existence proof Perhaps mkYear returns an error if the argument is not a four digit year. The point is to make it difficult to make appointments for times that occured during the Roman Empire.
The type of hour minute and second should protect the user from 28 o'clock and making appointments for 31:101 PM So we probably want:
data Hour = H0 | H1 | H2 ... H23 data Minute = M0 | M1 | M2 ... M59 data Second = S0 | S1 | S2 ... S59
Again, it's simpler to use actual numbers. These values almost never appear as literals in a program, so they would have to be converted to/from numbers anyway, which is unnecessary and no other language does this.
Its only simpler to use actual numbers if the library does not provide the services to make it easy to use these values. And we should certainly prefer to produce errors at constructor time rather than at destructor time. It should not be possible to construct a Time like 43:84:00.
There are also lots of contexts where you want a time but not a date e.g. an alarm clock....
And there are contexts where you want a weekday and hour/minute but not a month or second (a weekly schedule), contexts where you want month/day but not a year (a holiday with a fixed date) etc. Where to stop?
Are you seriously saying that you never have use for a Date without a Time? On what date were you born? In my particular case, I am storing a lot of dates and don't also want to consumer memory for Time data that I won't use. Note: I would grant that Time without Date is a lot less useful but it seems like the way to go once you have a Date type. Perl's incomplete type misses the point I am making here. -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com