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Today's Topics:
1. Date type needs helper function? (Galaxy Being)
2. Re: Date type needs helper function? (Matthew Low)
----------------------------------------------------------------------
Message: 1
Date: Thu, 1 Jul 2021 11:41:21 -0500
From: Galaxy Being <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: [Haskell-beginners] Date type needs helper function?
Message-ID:
<cafahfsvei93bdfau_n0mhgebs_lqtumjonq9_am3bljoyfz...@mail.gmail.com>
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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
[email protected]
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Message: 2
Date: Thu, 1 Jul 2021 11:41:11 -0600
From: Matthew Low <[email protected]>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <[email protected]>
Subject: Re: [Haskell-beginners] Date type needs helper function?
Message-ID:
<CAC=gtkz+v90t-r5hemw3kc5os_9i0frnujnhofjanu8rrgt...@mail.gmail.com>
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> 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 <[email protected]> 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
> [email protected]
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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