[Haskell-cafe] Using Haskell types in a database

[Adam Bergmark]

You shouldn't be using derivePersistField here if you want another db type.
As you say there is a predefined mapping but you can override this by
defining custom instances for your newtype.


On Tue, Feb 10, 2015 at 7:11 PM, Bob Ippolito <bob at redivi.com> wrote:

> If you're using Postgres you can use types in the database
> http://www.postgresql.org/docs/9.2/static/sql-createtype.html
>
> I don't know what these look like from the Haskell drivers but you can
> have typed SQL if you want it.
>
>
> On Tuesday, February 10, 2015, Nikita Karetnikov <nikita at karetnikov.org>
> wrote:
>
>> > That part I don't understand.  Why does your proposal require varchars
>> > everywhere?
>>
>> That's what Persistent generates (using derivePersistField) for the
>> second table.  For example, the Haskell value
>>
>> MyText "foo"
>>
>> where MyText is
>>
>> newtype MyText = MyText Text deriving (Show, Read)
>> derivePersistField "MyText"
>>
>> will be stored in PostgreSQL as a varchar
>>
>> 'MyText "foo"'
>>
>> I assume that Persistent (or its PostgreSQL backend) has a predefined
>> mapping from a commonly used Haskell types to the database types.
>> And everything else is represented as a varchar.
>>
>> So there are two choices, corresponding to the two tables shown in the
>> original message:
>>
>> 1. Design tables with the (basic) database types in mind (integer,
>>    boolean, timestamp, varchar, etc.) then wrap and unwrap values each
>>    time you interact with the database.
>>
>> 2. Use Haskell types like MyText or something more complex directly
>>    and end up with varchar everywhere.
>>
>> I wonder how bad is the second option in practice because
>> wrapping/unwrapping is error-prone.  Of course, the varchar thing makes
>> the alarm bells ring in my head, but when does it significantly affect
>> performance?  For example, maybe it's okay for a small shop, or is it a
>> no go for everyone?
>>
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20150210/24cb6956/attachment.html>