Yesod处理程序，发布文件的内容 [英] Yesod handlers, content of POSTed files

问题描述

  postImportR = do 
 fi<  -  lookupFiles文件
 fc<  -  lift $ fileSource（fi !! 0）$$ consume 
  

似乎可行至少我可以liftIO $ print fc），将它分解为一个迭代函数不会：

  process :: [FileInfo]  - > [String] 
 process [] = [] 
 process（f：r）= do 
 fn < -  fileName f 
 fc < -  lift $ fileSource f $$消耗
（[fn]：（process r））
 
 postImportR = do 
 fi<  -  lookupFilesfile
 process fi 
  

甚至可以使用lambda函数：

<$ p （文件名f，提取$ fileSource f $$消耗））fi

在Handler中它给我一个类型错误，我不明白。

我的错在哪里 - - 喜欢从文件的行（和l生成数据库导入内容赚取更多的Haskell，当然）。

解决方案

您有

  fileName :: FileInfo  - >文本
  

所以你不能直接使用 fileName 在一个do-block中，比如

  fn < -  fileName f 
  pre> 
 
 
这需要是一个绑定的绑定 
 
 
  let fn = fileName f 
  
接下来就是 process :: [FileInfo ]  - > [String] 不可能（1），
  fileSource :: FileInfo - >源（ResourceT IO）ByteString 
  
所以带有 
 
 
  fc<  -  lift $ fileSource f $$ consume 
  
你在一个do块中，它的 Monad 是 MonadIO ，你不能从 Monad ，它可以包装任意 IO  --actions，就像你不能脱离 IO 本身。
 
 
 
您可以拥有的是
 
 
  process ::（SomeFiendishConstraint m）=> [FileInfo]  - > m [Text] 
 process [] = return [] 
 process（f：r）= do 
 let fn = fileName f 
 lift $ fileSource f $$ consume 
 fs<  - 进程r 
返回（fn：fs）
  
或者更简洁，
 
 
  process = mapM（\f  - > lift $ fileSource f $$ consume>> return fileName f） 
  
然后 
 
 
  postImportR = do 
 fi<  -  lookupFilesfile
 process fi 
  
 （1）限制 unsafePerformIO 。
 
while the following code:
postImportR = do
    fi <- lookupFiles "file"
    fc <- lift $ fileSource (fi !! 0) $$ consume
seems to work (at least can I "liftIO $ print fc), splitting it off to a function for iterating doesn't:
process :: [FileInfo] -> [String]
process [] = []
process (f:r) = do
    fn <- fileName f
    fc <- lift $ fileSource f $$ consume
    ([fn] : (process r))

postImportR = do
    fi <- lookupFiles "file"
    process fi
or even with a lambda function:
files <- L.map (\f -> (fileName f, lift $ fileSource f $$ consume)) fi
in the Handler it gives me a type error I don't understand.

Where's my fault -- liked to generate content for database import from the file's lines (and to learn some more Haskell, of course).
解决方案
You have
fileName :: FileInfo -> Text
so you can't directly use fileName in a do-block like
fn <- fileName f
That would need to be a let-binding
let fn = fileName f
The next thing is what makes process :: [FileInfo] -> [String] impossible(1), 
fileSource :: FileInfo -> Source (ResourceT IO) ByteString
so with
fc <- lift $ fileSource f $$ consume
you are in a do-block whose Monad is a MonadIO, and you can't get out of a Monad that can wrap arbitrary IO-actions, just like you can't get out of IO itself.

What you can have is
process :: (SomeFiendishConstraint m) => [FileInfo] -> m [Text]
process [] = return []
process (f:r) = do
    let fn = fileName f
    lift $ fileSource f $$ consume
    fs <- process r
    return (fn : fs)
or, more succinctly,
process = mapM (\f -> lift $ fileSource f $$ consume >> return fileName f)
and then
postImportR = do
    fi <- lookupFiles "file"
    process fi
(1) Barring unsafePerformIO.

                        
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