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
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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