postgresql-简单的查询错误 [英] postgresql-simple query error

问题描述

编辑：我现在有一个更好的想法是什么问题。当我在旧的 psql 中执行该查询时，我得到以下结果：

  lwm @ verbos 
 =＃SELECT * FROM gerund LIMIT1; 
不定式│gerund│gerund_english 
────────────┼───────────────────────── ───────
放弃│放弃│放弃
  

所以， 3串？但是，我说我回来 IO [Only String] 。我确信这是我的类型签名，这是搞砸的事情...

我试图使用 postgresql-简单的库包含 Haskell 。我的代码与我看到的错误一起粘贴在下面。任何人有任何想法？

我的数据库被称为 verbos 并且在其中有一个名为动名词。我能够运行一个查询_ ，它包含： connSELECT 2 + 2，并且工作正常。我也可以使用缺省信息（password ='postgres'： psql -h localhost -p 5432 -U postgres （来自docs [ 1
$ b $ pre $ { - ＃LANGUAGE OverloadedStrings＃ - }

模块Main其中

import Control.Monad
import Control.Applicative
import Database.PostgreSQL.Simple

main = do
conn< - connect defaultConnectInfo {
connectPassword =postgres，
connectDatabase =verbos
}
mapM_ print =<（query_connSELECT * FROM gerund LIMIT 1:: IO [ ）


给我以下错误：

ConversionFailed {errSQLType =3 values：[（Basic {typoid = Oid 1043，
typcategory =' S'，typdelim ='，'，typname = \varchar \}，只需
\abandonar \），（Basic {typoid = Oid 1043，typcategory ='S'，typdelim
='，'，typname = \varchar \}，Just \abandonando\），（Basic {typoid = Oid 1043，typcategory ='S'，typdelim ='，'，typname =
\varchar \}，只是\abandoning \）]，errSQLTableOid = Nothing，
errSQLField =，errHaskellType =目标类型中有1个槽位，
errMessage =目标类型转换的列数与
的数目不匹配} b $ b

解决方案

好的，感谢@AlpMestanogullari，@ muistooshort，我在这里得到了答案。我的最终代码是：

  { - ＃LANGUAGE OverloadedStrings＃ - } 
 
模块Main其中
 
导入Control.Applicative 
导入Database.PostgreSQL.Simple 
导入Database.PostgreSQL.Simple.FromRow 
 
数据Gerund = Gerund {
f： ：String，
s :: String，
t :: String 
}派生（显示）
 
实例FromRow Gerund其中
 fromRow = Gerund< $> ;字段< *>字段< *>字段
 
 main = do 
 conn<  -  connect defaultConnectInfo {
 connectPassword =postgres，
 connectDatabase =verbos
} 
 mapM_print =<< （query_conn q :: IO [Gerund]）
 where q =SELECT * FROM gerund LIMIT 1
  

注意：

• 知道我的结果包含3个结果列，我需要定义一个类型，结果有'空格'（ f ， s 和<$ c $在 Gerund 类型中）


• 我遵循文档[ 1 ]对于FromRow密切合作我定义了类型和实例。


• 您需要导入 import Database.PostgreSQL.Simple.FromRow 才能访问字段。

EDIT: I now have a better idea of what is going wrong. When I perform that query in plain old psql, I get the following result:

lwm@verbos
=# SELECT * FROM gerund LIMIT1;                 
  infinitive │   gerund    │ gerund_english 
 ────────────┼─────────────┼────────────────
  abandonar  │ abandonando │ abandoning

So, I am getting back 3 strings? However, I say that I am getting back IO [Only String]. I am sure it is my type signature here that is messing things up ...

I am trying to make a simple query using the postgresql-simple library with Haskell. My code is pasted below along with the error I am seeing. Anyone got any ideas?

My database is called verbos and within it, I have a table called gerund. I am able to run a query_ that contains: conn "SELECT 2 + 2" and that works fine. I can also connect to my database with the default data as specified with the default information (password = 'postgres' : psql -h localhost -p 5432 -U postgres (from the docs[1])

{-# LANGUAGE OverloadedStrings #-}

module Main where

import Control.Monad
import Control.Applicative
import Database.PostgreSQL.Simple

main = do
    conn <- connect defaultConnectInfo {
        connectPassword = "postgres",
        connectDatabase = "verbos"
    }
    mapM_ print =<< (query_ conn "SELECT * FROM gerund LIMIT 1" :: IO [Only String])

Gives me the following error:

ConversionFailed {errSQLType = "3 values: [(Basic {typoid = Oid 1043, typcategory = 'S', typdelim = ',', typname = \"varchar\"},Just \"abandonar\"),(Basic {typoid = Oid 1043, typcategory = 'S', typdelim = ',', typname = \"varchar\"},Just \"abandonando\"),(Basic {typoid = Oid 1043, typcategory = 'S', typdelim = ',', typname = \"varchar\"},Just \"abandoning\")]", errSQLTableOid = Nothing, errSQLField = "", errHaskellType = "1 slots in target type", errMessage = "mismatch between number of columns to convert and number in target type"}

解决方案

OK, Thanks to @AlpMestanogullari, @muistooshort, I got an answer here. My final code is:

{-# LANGUAGE OverloadedStrings #-}

module Main where

import Control.Applicative
import Database.PostgreSQL.Simple
import Database.PostgreSQL.Simple.FromRow

data Gerund = Gerund { 
    f :: String, 
    s :: String, 
    t :: String 
} deriving (Show)

instance FromRow Gerund where 
    fromRow = Gerund <$> field <*> field <*> field

main = do
    conn <- connect defaultConnectInfo {
        connectPassword = "postgres",
        connectDatabase = "verbos"
    }
    mapM_ print =<< (query_ conn q :: IO [Gerund])
        where q = "SELECT * FROM gerund LIMIT 1"

Notes:

• Knowing that my result contained 3 result columns, I needed to define a type that had 'space' for the results (f, s and t in the Gerund type)
• I followed the docs[1] for FromRow closely to get my type and instance defined.
• You need to import import Database.PostgreSQL.Simple.FromRow to access things like field.

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