Re: [Haskell-cafe] cgi liftM liftIO

14 Jun 2008


      I think you need to put liftIO in front of the IO actions you want to  
do inside the CGI Monad. Like in this example
...
http://www.haskell.org/haskellwiki/ 
Practical_web_programming_in_Haskell#File_uploads
(Why did I need to use google to find that? The wiki search in awful.  
Searching for CGI returns nothing, whereas with google the above is  
the first hit.)

Am 13.06.2008 um 15:41 schrieb Cetin Sert:
...
Hi,
Could someone please care to explain what I am doing wrong below in  
cgiMain2 and how can I fix it?
./Main.hs:25:15:
    No instance for (MonadCGI IO)
      arising from a use of `output' at ./Main.hs:25:15-20
    Possible fix: add an instance declaration for (MonadCGI IO)
    In the first argument of `($)', namely `output'
    In the expression: output $ renderHtml $ page "import" fileForm
    In the definition of `upload':
        upload = output $ renderHtml $ page "import" fileForm
./Main.hs:57:29:
    Couldn't match expected type `CGI CGIResult'
           against inferred type `IO CGIResult'
    In the first argument of `handleErrors', namely `cgiMain2'
    In the second argument of `($)', namely `handleErrors cgiMain2'
    In the expression: runCGI $ handleErrors cgiMain2
import IO
import Network.CGI
import Text.XHtml
import qualified Data.ByteString.Lazy as BS
import Control.Monad (liftM)
import Data.Maybe (fromJust)
import Interact
fileForm = form ! [method "post", enctype "multipart/form-data"] <<
             [afile "file", submit "" "Upload"]
page t b = header << thetitle << t +++ body << b
cgiMain1 = do
  getInputFPS "file" ↠ λms → maybe upload contents ms ↠ return
  where
    upload   = output $ renderHtml $ page "import" fileForm
    contents = outputFPS
cgiMain2 = do
  getInputFPS "file" ↠ λms → maybe upload contents ms ↠ return
  where
    upload   = output $ renderHtml $ page "import" fileForm
    contents = λs → do
                      (i,o,h,_) ← runUnzip
                      BS.hPutStr i s
                      c ← BS.hGetContents o
                      outputFPS c
{-
                      (i,o,h,_) ← runUnzip
                      BS.hPutStr i s
                      BS.hGetContents o ↠ outputFPS
-}
{-
liftM :: (Monad m) => (a1 -> r) -> m a1 -> m r
liftIO :: (MonadIO m) => IO a -> m a
saveFile n =
    do cont <- liftM fromJust $ getInputFPS "file"
       let f = uploadDir ++ "/" ++ basename n
       liftIO $ BS.writeFile f cont
       return $ paragraph << ("Saved as " +++ anchor ! [href f] <<  
f +++ ".")
-}
runUnzip = runInteractiveCommand "unzip -l /dev/stdin"
main = runCGI $ handleErrors cgiMain2
Best Regards,
Cetin Sert
P/s: what are lifts o_O?
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Re: [Haskell-cafe] cgi liftM liftIO

Adrian Neumann