How to get primary key type?

[yaitskov]

Hi,

I am trying to define a servant endpoint returning an entity by primary key.
If id type is hardcoded as Int32 then the following snippet type checks:

data UserTableT f
  = UserTable
    { name :: C f Text -- e.g. value aggregate over the hour
    , id :: C f Int32  -- e.g. hour in day
    } deriving (Generic, Beamable)

type UserTableId = PrimaryKey UserTableT Identity

instance Table UserTableT where
  data PrimaryKey UserTableT f
    = UserTableId (C f Int32) deriving (Generic, Beamable)
  primaryKey = UserTableId . idT

showUserTable :: Int32 -> AppM UserTable
showUserTable utId = do
  c <- asks dbConnection'
  utM <- liftIO $
    runBeamPostgres c $
      runSelectReturningOne $
        select $ do
          t <- all_ (_comajorUserTables comajorDb)
          guard_ (idT t ==. val_ utId )
          pure t

but

showUserTable :: PrimaryKey UserTableT Identity -> AppM UserTable
showUserTable utId = do

I guess it is expected, so how get pure Int32 out of beam table type?

[KAction]

I am not sure what AppM is supposed to be, but to get underlying Int32 can be extracted by pattern matching:

foo : PrimaryKey UserTableT Identity -> Int32
foo (UserTableId x) = x

I find it more convenient to define primary keys as single-field records instead:

data PrimaryKey UserTableT f = UserTableId { unwrapUserTableId :: C f Int32 } deriving (Generic, Beamable)

NB: I am learning to use beam for just a week, there may be better idioms.