Haskell：使用 postgresql.simple 获取 table count(*)（使用连接池）

Question

拜托，..我怎样才能从一个 table 中得到 count(*)？我正在尝试连接一些代码片段，..但到目前为止没有成功。

这是我的：

fetchSimple :: FromRow r => Pool Connection -> Query -> IO [r]
fetchSimple pool sql = withResource pool retrieve
   where retrieve conn = query_ conn sql 

getUsersCount :: Pool Connection -> IO (Int)
getUsersCount pool = do
   res <- fetchSimple pool "SELECT count(*) FROM article" :: IO [Int]
   let f = head res
   return f

这给了我以下错误：

No instance for (FromRow Int) arising from a use of ‘fetchSimple’
In a stmt of a 'do' block:
  res <- fetchSimple pool "SELECT count(*) FROM article" :: IO [Int]
In the expression:
  do { res <- fetchSimple pool "SELECT count(*) FROM article" ::
                IO [Int];
       let f = head res;
       return f }
In an equation for ‘getUsersCount’:
    getUsersCount pool
      = do { res <- fetchSimple pool "SELECT count(*) FROM article" ::
                      IO [Int];
             let f = ...;
             return f }

Answer 1

如果您只想提取单个值，则不要直接这样做。您将其包装在 Only.

中

所以尝试注释 :: IO [Only Int]

Haskell：使用 postgresql.simple 获取 table count(*)（使用连接池）

Haskell: get table count(*) with postgresql.simple (with Connection Pool)

postgresql

haskell

connection-pooling