如何以模块化方式构成Database.Esqueleto查询，以便在定义“基本”查询和相应的结果集之后，可以通过添加其他内部联接和where表达式来限制结果集。

另外，由于基本查询不是照此执行的，而是使用LIMIT和OFFSET的修改版本，因此如何将返回实体列表（或字段元组）的基本查询转换为对结果集进行计数的查询。

Yesod书中采用的以下不正确的Haskell代码段有望阐明我的目标。

{-# LANGUAGE QuasiQuotes, TemplateHaskell, TypeFamilies, OverloadedStrings #-}
{-# LANGUAGE GADTs, FlexibleContexts #-}
import qualified Database.Persist as P
import qualified Database.Persist.Sqlite as PS
import Database.Persist.TH
import Control.Monad.IO.Class (liftIO)
import Data.Conduit
import Control.Monad.Logger
import Database.Esqueleto
import Control.Applicative

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
Person
    name String
    age Int Maybe
    deriving Show
BlogPost
    title String
    authorId PersonId
    deriving Show
Comment
    comment String
    blogPostId BlogPostId
|]

main :: IO ()
main = runStdoutLoggingT $ runResourceT $ PS.withSqliteConn ":memory:" $ PS.runSqlConn $ do
    runMigration migrateAll

    johnId <- P.insert $ Person "John Doe" $ Just 35
    janeId <- P.insert $ Person "Jane Doe" Nothing

    jackId <- P.insert $ Person "Jack Black" $ Just 45
    jillId <- P.insert $ Person "Jill Black" Nothing

    blogPostId <- P.insert $ BlogPost "My fr1st p0st" johnId
    P.insert $ BlogPost "One more for good measure" johnId
    P.insert $ BlogPost "Jane's" janeId

    P.insert $聽Comment "great!" blogPostId

    let baseQuery = select $ from $ \(p `InnerJoin` b) -> do聽
        on (p ^. PersonId ==. b ^. BlogPostAuthorId)
        where_ (p ^. PersonName `like` (val "J%"))
        return (p,b)

    -- Does not compile
    let baseQueryLimited = (,) <$> baseQuery <*> (limit 2)

    -- Does not compile
    let countingQuery = (,) <$> baseQuery <*> (return countRows)

    -- Results in invalid SQL 
    let commentsQuery = (,) <$> baseQuery
                <*> (select $聽from $ \(b `InnerJoin` c) -> do
                        on (b ^. BlogPostId ==. c ^. CommentBlogPostId)
                        return ())

    somePosts <- baseQueryLimited
    count <- countingQuery
    withComments <- commentsQuery
    liftIO $ print somePosts
    liftIO $ print ((head count) :: Value Int)
    liftIO $ print withComments
    return ()