Esqueleto只返回一列

我对Haskell还是个新手,目前正在尽我最大的努力防止我的大脑从内到外。 我用这种方式定义了一个非常简单的数据库模式:

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
  Instrument
    ticker String
    source String
    exchange String
    deriving Eq Show
|]

我注意到我可以通过以下方式查询表中的所有条目:

getAllInstruments :: (MonadIO m, MonadLogger m) => SqlReadT m [Entity Instrument]
getAllInstruments = select $ from $ \instrument -> do return instrument

但是,如果我只想得到一个专栏呢?我试过这样的方法:

-- doesn't compile
getAllTickers :: (MonadIO m, MonadLogger m) => SqlReadT m [String]
getAllTickers = select $ from $ \instrument -> do return (Value (instrument ^. ticker))

• Couldn't match expected type ‘EntityField val0 typ0’
  with actual type ‘StockPrice -> Ticker’

关于如何返回列的子集,我发现了一个类似的问题: Haskell Esqueleto project subset of columns to list of custom records

多亏了这个提示,我终于想出了以下代码:

getAllTickers' :: (MonadIO m, MonadLogger m) => SqlReadT m [String]
getAllTickers' = do 
  res <- select $ from $ \i -> return (i ^. InstrumentTicker)
  return $ map unValue res

-- Doesn't compile - need to enter with the map into the m***d - how ?
getAllTickers'' = map unValue (select $ from $ \i -> return (i ^. InstrumentTicker))