Haskell：Yesod 和 Esqueleto

Posted 2023-04-01

【中文标题】Haskell：Yesod 和 Esqueleto

【英文标题】：Haskell: Yesod and Esqueleto

【发布时间】：2014-03-29 04:50:39

【问题描述】：

如何让 Yesod 1.2 中的 runDB 与 Esqueleto 一起工作？

import Database.Esqueleto as E
getMainR :: Handler html
getMainR = do
        leases <- runDB $ E.select $ 
                          E.from $ \l -> do
                          E.where_ (l E.^. LeaseId E.==. E.val 1)
                          return l

        defaultLayout $ do
          setTitle "Main"

这是错误输出：

Handler/Main.hs:41:27:
    Couldn't match type `persistent-1.3.0.6:Database.Persist.Sql.Types.SqlPersistT'
                  with `SqlPersistT'
    Expected type: YesodDB App [E.Entity val0]
      Actual type: SqlPersistT (HandlerT App IO) [E.Entity val0]
    In the second argument of `($)', namely
      `select
       $ from
         $ \ l
             -> do  where_ (l ^. LeaseId ==. val 1);
                     return l '
    In a stmt of a 'do' block:
      leases <- runDB
                $ select
                  $ from
                    $ \ l
                        -> do  where_ (l ^. LeaseId ==. val 1);
                                return l 
    In the expression:
      do  leases <- runDB $ select $ from $ \ l -> do  ... ;
           defaultLayout $ do  setTitle "Tenants"  

Handler/Main.hs:42:41:
    Couldn't match expected type `E.EntityField val0 typ0'
                with actual type `Import.EntityField
                                    Lease
                                    (Import.KeyBackend
                                       persistent-1.3.0.6:Database.Persist.Sql.Types.SqlBackend
                                       Lease)'
    In the second argument of `(^.)', namely `LeaseId'
    In the first argument of `(==.)', namely `l ^. LeaseId'
    In the first argument of `where_', namely
      `(l ^. LeaseId ==. val 1)'

我试图按照本教程进行操作，但似乎他们的 runDB 设置与 Yesod 脚手架的设置方式不同。 https://github.com/yesodweb/yesod/wiki/Multi-select-field-populated-from-database-(using-esqueleto)

【参考方案1】：

这是一个依赖问题。 Esqueleto 必须使用最新版本的 persistent 和 persistent-sqlite 编译。