[Haskell-cafe] How to pass a polymorphic function in a record?

[adam vogt]

Hi Vlatko,

Did you consider:

{-# LANGUAGE RankNTypes #-}
data ThingCfg m = ThingCfg {
    thingDb  :: Text,
    thingRun_ :: forall a. Text -> m a -> IO a }

thingRun (ThingCfg db f) = f db

Maybe the `m' above should be SqlPersistM, if all your other backends use
that type.

--
Adam


On Tue, Jan 28, 2014 at 1:37 PM, Vlatko Basic <vlatko.basic at gmail.com>wrote:

> Hello Cafe,
>
> I'm playing with Persistent and have modules that I'd like to use on
> several backends. This is simplified situation.
>
> In shared module:
>
>   sqliteRun, postgresRun :: Text -> Int -> (ConnectionPool -> IO a) -> IO a
>   sqliteRun          = withSqlitePool
>   postgresRun conStr = withPostgresqlPool (encodeUtf8 conStr)
>
>   sqlRun :: Text -> Int -> SqlPersistM a -> IO a
>   sqlRun conStr poolSize = postgresRun conStr poolSize . runSqlPersistMPool
>   --sqlRun conStr poolSize = sqliteRun conStr poolSize . runSqlPersistMPool
>
> All works well if either 'sqlRun' above is commented/uncommented:
>
>
> In one of modules:
>
>   data ThingCfg = ThingCfg { thingDb :: Text }
>
>   listThings :: ThingCfg -> IO [Thing]
>   listThings db = sqlRun (thingDb db) $ selectList ...
>
>   findThing :: ThingId -> ThingCfg -> IO (Maybe Thing)
>   findThing uid db = sqlRun (thingDb db) $ getBy ...
>
>
>
> On call site simply:
>   let tdb = ThingCfg "test"
>   ts <- listThings tdb
>
>
> I would like to specify 'sqliteRun' or 'postgresRun' function as (some)
> parameter on the call site, but do not know how.
> Something of imaginary solution:
>
>   data ThingCfg = ThingCfg {
>       thingDb  :: Text,
>       thingRun :: SqlPersistM a -> IO a
>     }
>
> On call site:
>   let tdb = ThingCfg "test" sqliteRun
>   ts <- listThings tdb
>
> I want to keep it as an init param because there are other backends (class
> instances) that are not Persistent, so the use of 'sqlRun' on call site is
> not an option.
>
>
> What would be the best/correct way(s) to achieve that?
>
>
> Best regards,
>   Vlatko
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