[Haskell-beginners] SQLite3 Row <-> Data models -- Is this a typical Haskell pattern ?

Antoine Latter aslatter at gmail.com
Sun Mar 20 01:43:41 CET 2011

On Sat, Mar 19, 2011 at 7:21 PM, Stef T <stef at ummon.com> wrote:
> Hey Everyone,
>    Greetings, I am a total newb to haskell, so, I have a strange question (perhaps). Consider I have a data model which looks like this;
> data Brand = Brand { id :: Int,
>                                       name :: String,
>                                       created_at :: String,
>                                       updated_at :: String
>                                    }
>    Now, I have a sqlite3 function such as
> checkout :: Int -> IO (Either String [[Row Value]])
> checkout a = do
>       handle <- openConnection "/Users/stef/haskell/db/development.sqlite3"
>       execStatement handle $ "SELECT * from brands where id = " ++ show a
>    From this, I get a Row/Tuple. So far, so good, but, the question _I_ have is, how do you go about mapping a returned row to the data model ? Is this even a design pattern that is used in FP languages ? I admit, I have spent numerous years in the MVC world, so, perhaps this is simply "not done". It would seem to be a much nicer thing to then do ;

What problem are you running into trying to do this?

You would need to write functions to convert each column into the
format you want it for the Brand type, and then pass output of the
functions Brand data constructor. Depending on the format of the data
and the library you're using these functions might turn out to be
pretty simple.

It's considered good in Haskell to get data out of "weak" types into
specific types when you want them, as in from a String or from a 'Row
Value' - it makes manipulations of the values easier to read and you
get the compiler's help figuring things out.

Which SQL library are you using?

>    name myBrandModel
>    Thanks for reading this far, feel free to complain about my design :D
>    Regards
>    S.
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