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

[emacstheviking]

David,

At times like this I think am not even fit to code PHP for my day job.
I am going to have to read that very carefully when I wake up tomorrow.

Thanks.



On 10 March 2013 22:59, David McBride <toad3k at gmail.com> wrote:

> 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 at 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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>
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