> why can Book take everything naked but my Day version not?

The Day data type is actually just a newtype wrapper around an Int, which encodes the number of days since some point in the past - the docs more details: https://hackage.haskell.org/package/time-1.12/docs/Data-Time-Calendar.html. So to use the data constructor directly as in the Book example, you'd have to give it the number of days directly, something like ModifiedJulianDay 1234. The helper function gives a more convenient way to construct a Day. I believe this general approach of having functions to help you construct a data type is called 'smart constructors'. They're more common when the data constructors are not exported from a module, so the helper functions are the only way to create a value of that type - useful if you have some constraints you need enforced on your type.

On Thu, Jul 1, 2021 at 10:41 AM Galaxy Being <borgauf@gmail.com> wrote:
I am using the built-in data type Day (Data.Time) in two ways

data PDate = PDate Day

or

type PDate Day

doesn't seem to matter. But then this doesn't work

testrec1 = PDate 2021 7 1

I always must use the "helper function" fromGregorian

testrec0 = PDate (fromGregorian 2021 7 1)
...
PDate 2021-07-01

Looking at Real World Haskell examples

data BookInfo = Book Int String [String]  deriving (Show)
...
myInfo = Book 9780135072455 "Algebra of Programming" ["Richard Bird", "Oege de Moor"]

I know there's a great Haskell lesson to learn here, so why can Book take everything naked but my Day version not?


⨽
Lawrence Bottorff
Grand Marais, MN, USA
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