Your ipCamExec is IO (), but you are running it in the CGI a monad which is a type alias for CGIT IO a.  CGIT is an instance of MonadIO, so try liftIO ipCamExec.  liftIO has a type MonadIO m => IO a -> m a, which means that if you replace m with CGIT IO, you would get IO a -> CGIT IO a, which is exactly what you need.

On Sun, Mar 10, 2013 at 6:40 PM, emacstheviking <objitsu@gmail.com> wrote:

I am writing a stop-motion capture application using AngularJS and it's going OK. I was inspired to do so after installing "IPCamera" on my phone and Sony tablet. A typical IPCamera session lives on an internal address like this, this example will turn on the LED on the camera:

   http://192.168.0.5:8080/enabletorch

Just because I can (or so I thought), I decided to write a tiny little FastCGI application in Haskell to act as a proxy using the PATH_INFO variable. This means that in to my Javascript code I have this code in a service file:

angular.module('stomoServices', ['ngResource']).

    factory(

'IPCamera',

function($resource, urlIPCameraAPI) {

   return $resource(

urlIPCameraAPI,

{}, {

   ledOn:    { method: 'GET', params: {featureReq: 'enabletorch' }},

   ledOff:   { method: 'GET', params: {featureReq: 'disabletorch' }},

   focusOn:  { method: 'GET', params: {featureReq: 'focus' }},

   focusOff: { method: 'GET', params: {featureReq: 'nofocus'}}

});

});

and I then issue commands like "IPCamera.ledOn()" etc. All very nice except that it doesn't work yet because I can't get the worlds seemingly simplest CGI application to compile yet! Here is the code that I have, it could be "cleared up" but this is what I have so far:

main :: IO ()

main = runFastCGI . handleErrors $ do

  command <- getVar "PATH_INFO"

  case command of

    Nothing  ->

      outputError 400 "Missing IPCamera instruction (PATH_INFO)" []

    Just cmd ->

      ipCamExec (tail cmd) >> output "OK" -- tail drops the "/"

      where

        ipCamExec :: String -> IO ()

        ipCamExec url = do

          simpleHTTP (getRequest url) -- don't want or need response.

          return () -- to match the return type or so I thought.

and the error message I cannot seem to understand as it fills me with monadic fear which I can't get out of:

ipcamera.hs:16:7:

    Couldn't match expected type `CGIT IO a0' with actual type `IO ()'

    In the return type of a call of `ipCamExec'

    In the first argument of `(>>)', namely `ipCamExec (tail cmd)'

    In the expression: ipCamExec (tail cmd) >> output "OK"

Please could some kind souls explain to me in simple terms just what is going on and why I am close to tears right now? I have read the definitions of CGIResult and CGI and they leave me cold. I am trying to understand monads more but at times like this I once again realise what a complete rank beginner I am!

Thanks.

Sean.

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