The lookFile function is used to extract an uploaded file:
#ifdef HsColour
> lookFile :: String -> RqData (FilePath, FilePath, ContentType)
#endif
It returns three values:
- The location of the temporary file which holds the contents of the file
- The local filename supplied by the browser. This is typically the name of the file on the users system.
- The content-type of the file (as supplied by the browser)
The temporary file will be automatically deleted after the Response is sent. Therefore, it is essential that you move the file from the temporary location.
In order for file uploads to work correctly, it is also essential that your <form> element contains the attributes enctype="multipart/form-data" and method="POST"
The following example has a form which allows a user to upload a file. We then show the temporary file name, the uploaded file name, and the content-type of the file. In a real application, the code should use System.Directory.renameFile (or similar) to move the temporary file to a permanent location. This example looks a bit long, but most of the code is just HTML generation using BlazeHtml. The only really new part is the use of the lookFile function. Everything else should already have been covered in previous sections. So if you don't understand something, try looking in earlier material.
> {-# LANGUAGE OverloadedStrings #-}
> import Control.Monad (msum)
> import Happstack.Server (Response, ServerPart, defaultBodyPolicy, decodeBody, dir
> , lookFile, nullConf, ok, simpleHTTP, toResponse)
> import Text.Blaze as B
> import Text.Blaze.Html4.Strict as B hiding (map)
> import Text.Blaze.Html4.Strict.Attributes as B hiding (dir, title)
>
> main :: IO ()
> main = simpleHTTP nullConf $ upload
>
> upload :: ServerPart Response
> upload =
> do decodeBody (defaultBodyPolicy "/tmp/" (10*10^6) 1000 1000)
> msum [ dir "post" $ post
> , uploadForm
> ]
>
> uploadForm :: ServerPart Response
> uploadForm = ok $ toResponse $
> html $ do
> B.head $ do
> title "Upload Form"
> B.body $ do
> form ! enctype "multipart/form-data" ! B.method "POST" ! action "/post" $ do
> input ! type_ "file" ! name "file_upload" ! size "40"
> input ! type_ "submit" ! value "upload"
>
> post :: ServerPart Response
> post =
> do r <- lookFile "file_upload"
> ok $ toResponse $
> html $ do
> B.head $ do
> title "Post Data"
> B.body $ mkBody r
> where
> mkBody (tmpFile, uploadName, contentType) = do
> p (toHtml $ "temporary file: " ++ tmpFile)
> p (toHtml $ "uploaded name: " ++ uploadName)
> p (toHtml $ "content-type: " ++ show contentType)
[Source code for the app is here.]
Remember that you must move the temporary file to a new location or it will be garbage collected after the Response is sent. In the example code we do not move the file, so it is automatically deleted.