如何在Haskell (使用Vinyl)中泛化Opaleye查询？

Question

我的问题是在下面的代码块中的巨大横幅之间。

请原谅代码转储，对于任何想要复制的人来说，这都是粘贴在这里的，而且这段代码确实像预期的那样工作，尽管有点奇怪。注意最后两行，它们打印正确的SQL。

目标：

我有带有Text类型主键的表，特别是电子邮件。我没有为每个表编写一个新的查询函数，而是承担了泛化该函数的任务，这样我就可以键入--安全地查询任何有电子邮件的表。

问题：

为了让它发挥作用，我必须包括：

instance Default Constant CEmail (Column PGText) where
  def = undefined

这让我觉得我做错了什么。对于建立一个可以从任何有电子邮件的表中找到记录的查询，有什么建议吗？

{- stack
--resolver lts-8.2
--install-ghc
exec ghci
--package aeson
--package composite-base
--package composite-aeson
--package text
--package string-conversions
--package postgres-simple
--package vinyl
-}

{-# LANGUAGE
Arrows
, DataKinds
, OverloadedStrings
, PatternSynonyms
, TypeOperators
, TemplateHaskell
, FlexibleContexts
, RankNTypes

, ConstraintKinds
, TypeSynonymInstances
, FlexibleInstances
, MultiParamTypeClasses
#-}

import Data.Vinyl (RElem)
import Data.Functor.Identity (Identity)
import Data.Vinyl.TypeLevel (RIndex)
import Composite.Aeson (JsonFormat, defaultJsonFormatRec, recJsonFormat, toJsonWithFormat)
import Composite.Opaleye (defaultRecTable)

import Composite.Record (Record, Rec(RNil), (:->), pattern (:*:))
import Composite.TH (withOpticsAndProxies)
import Control.Arrow (returnA)
import Control.Lens (view)
import Data.Int (Int64)
import Data.Proxy (Proxy(Proxy))
import Data.Text (Text)
import Opaleye
import Opaleye.Internal.TableMaker (ColumnMaker)
import Data.String.Conversions (cs)
import qualified Data.Aeson as Aeson

import qualified Database.PostgreSQL.Simple as PGS -- used for printSql
import Data.Profunctor.Product.Default (Default(def))


--------------------------------------------------
-- | Types


-- | Newtype ClearPassword so it can't be passed around as ordinary Text
newtype ClearPassword a = ClearPassword a

withOpticsAndProxies [d|
  type FEmail = "email" :-> Text
  type CEmail = "email" :-> Column PGText

  type FAge = "age" :-> Text
  type CAge = "age" :-> Column PGText

  type FClearPassword = "clearpass" :-> ClearPassword Text
  type CHashPassword = "hashpass" :-> Column PGText
  |]


--------------------------------------------------
-- | Db Setup

-- | Helper Fn
printSql :: Default Unpackspec a a => Query a -> IO ()
printSql = putStrLn . maybe "Empty query" id . showSqlForPostgres

-- | Db Records
type DbUser = '[CEmail, CAge]
type DbPassword = '[CEmail, CHashPassword]


--------------------------------------------------
--------------------------------------------------
--
-- LOOK HERE vvvvvvvvvvvvvvvvvvvvvvvv
--
--------------------------------------------------
--------------------------------------------------

type RecWith f rs = (Default ColumnMaker (Record rs) (Record rs),
                     Default Constant f (Column PGText),
                     RElem f rs (RIndex f rs))

-- | queryByEmail needs this, but totally works if `def` is declared
-- as `undefined` ???
instance Default Constant CEmail (Column PGText) where
  def = undefined

queryByEmail :: (RecWith CEmail rs) =>
                Table a (Record rs) -> Text -> QueryArr () (Record rs)
queryByEmail table email = proc () -> do
  u <- queryTable table -< ()
  let uEmail = view cEmail u
  restrict -< uEmail .=== constant email
  returnA -< u

--------------------------------------------------
--------------------------------------------------
--
-- LOOK UP ^^^^^^^^^^^^^^^^^^^^^^^^
--
--------------------------------------------------
--------------------------------------------------

userTable :: Table (Record DbUser) (Record DbUser)
userTable = Table "user" defaultRecTable

-- | Password
passwordTable :: Table (Record DbPassword) (Record DbPassword)
passwordTable = Table "password" defaultRecTable

-- SELECT ... FROM "user" ...
queryUserTest = printSql $ queryByEmail userTable "hi"

-- SELECT ... FROM "password" ...
queryPasswordTest = printSql $ queryByEmail passwordTable "hi"

Answer 1

删除无关的Default Constant f (Column PGTest)约束，您应该可以：

#!/usr/bin/env stack
{- stack --resolver lts-8.11 --install-ghc exec ghci --package aeson --package composite-base --package composite-aeson --package text --package string-conversions --package vinyl --package composite-opaleye -}
{-# LANGUAGE Arrows, DataKinds, OverloadedStrings, PatternSynonyms, TypeOperators, TemplateHaskell, FlexibleContexts, RankNTypes, ConstraintKinds, TypeSynonymInstances, FlexibleInstances, MultiParamTypeClasses #-}

import Composite.Opaleye (defaultRecTable)
import Composite.Record (Record, (:->))
import Composite.TH (withOpticsAndProxies)
import Control.Arrow (returnA)
import Control.Lens (view)
import Data.Profunctor.Product.Default (Default)
import Data.Text (Text)
import Data.Vinyl (RElem)
import Data.Vinyl.TypeLevel (RIndex)
import Opaleye.Internal.TableMaker (ColumnMaker)

import Opaleye


newtype ClearPassword a = ClearPassword a

withOpticsAndProxies [d|
  type FEmail = "email" :-> Text
  type CEmail = "email" :-> Column PGText

  type FAge = "age" :-> Text
  type CAge = "age" :-> Column PGText

  type FClearPassword = "clearpass" :-> ClearPassword Text
  type CHashPassword = "hashpass" :-> Column PGText
  |]

type DbUser = '[CEmail, CAge]
type DbPassword = '[CEmail, CHashPassword]

printSql :: Default Unpackspec a a => Query a -> IO ()
printSql = putStrLn . maybe "Empty query" id . showSqlForPostgres

queryByEmail :: (RElem CEmail rs (RIndex CEmail rs), Default ColumnMaker (Record rs) (Record rs)) => Table a (Record rs) -> Text -> QueryArr () (Record rs)
queryByEmail table email = proc () -> do
  u <- queryTable table -< ()
  let uEmail = view cEmail u
  restrict -< uEmail .=== constant email
  returnA -< u

userTable :: Table (Record DbUser) (Record DbUser)
userTable = Table "user" defaultRecTable

passwordTable :: Table (Record DbPassword) (Record DbPassword)
passwordTable = Table "password" defaultRecTable

queryUserTest = printSql $ queryByEmail userTable "hi"
queryPasswordTest = printSql $ queryByEmail passwordTable "hi"

constant email调用使用(已经存在的) Default Constant Text (Column PGText)约束；to email具有CEmail类型，相反，您将需要一个重要的、非定义的实例。