[Haskell-cafe] Using Haskell types in a database

Adam Bergmark adam at bergmark.nl
Tue Feb 10 18:53:36 UTC 2015

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?
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