[Haskell-beginners] Trouble in (Fast)CGI land: matching `CGIT IO a0' with actual type `IO ()'

emacstheviking objitsu at gmail.com
Sun Mar 10 23:40:04 CET 2013

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:

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']).
function($resource, urlIPCameraAPI) {
    return $resource(
 {}, {
    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 "/"
        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:

    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!

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/beginners/attachments/20130310/8e43f9c0/attachment.htm>

More information about the Beginners mailing list