Re: [Haskell-beginners] Beginners Digest, Vol 57, Issue 12

10 Mar 2013


      Enviado desde mi iPhone

El 10/03/2013, a las 18:00, beginners-request@haskell.org escribió:
...
Send Beginners mailing list submissions to
   beginners@haskell.org
To subscribe or unsubscribe via the World Wide Web, visit
   http://www.haskell.org/mailman/listinfo/beginners
or, via email, send a message with subject or body 'help' to
   beginners-request@haskell.org
You can reach the person managing the list at
   beginners-owner@haskell.org
When replying, please edit your Subject line so it is more specific
than "Re: Contents of Beginners digest..."
Today's Topics:
1. Re:  Create new value for given type (Kim-Ee Yeoh)
  2. Re:  Performance problem with Haskell/OpenGL/GLFW
     (Jesper S?rnesj?)
  3. Re:  Performance problem with Haskell/OpenGL/GLFW
     (Jesper S?rnesj?)
  4. Re:  Performance problem with Haskell/OpenGL/GLFW
     (Hollister Herhold)
  5.  Trouble in (Fast)CGI land: matching `CGIT IO    a0' with actual
     type `IO ()' (emacstheviking)
  6. Re:  Trouble in (Fast)CGI land: matching `CGIT IO a0' with
     actual type `IO ()' (David McBride)
----------------------------------------------------------------------
Message: 1
Date: Mon, 11 Mar 2013 03:48:33 +0700
From: Kim-Ee Yeoh 
Subject: Re: [Haskell-beginners] Create new value for given type
To: The Haskell-Beginners Mailing List - Discussion of primarily
   beginner-level topics related to Haskell 
Message-ID:
   
Content-Type: text/plain; charset=ISO-8859-1
On Mon, Mar 11, 2013 at 1:33 AM, Kim-Ee Yeoh  wrote:
...
class MyTheory a  where
 someKindOfChoice :: a -> [a] -> [a]
Whoops, that should be [a] -> [a], or perhaps more usefully [a] -> a.
In a way, such a function constructively /proves/ that the type is
infinite. Of course, it has to satisfy a bunch of conditions. You can
sort of see how going down this path leads naturally to Coq and Agda.
-- Kim-Ee
------------------------------
Message: 2
Date: Mon, 11 Mar 2013 08:11:36 +1100
From: Jesper S?rnesj? 
Subject: Re: [Haskell-beginners] Performance problem with
   Haskell/OpenGL/GLFW
To: The Haskell-Beginners Mailing List - Discussion of primarily
   beginner-level topics related to Haskell 
Message-ID:
   
Content-Type: text/plain; charset=ISO-8859-1
On Mon, Mar 11, 2013 at 1:31 AM, Andrey Yankin  wrote:
...
AFAIK glfw-b uses its own version of glfw which is built during setup.
There is a makefile inside the package.
GLFW-b does indeed bundle its own version of the GLFW C library [1],
version 2.7.3 as of right now. This is why it can be installed without
first installing the C library on your system.
However, this is not the library that Test2.hs (nor test2.c) links
against. In fact, that program does not use GLFW-b at all, but rather
simply declares a few entry points with C calling convention that
should be there at linking time. I did this only to be able to rule
out any problem with GLFW-b.
...
Can't reproduce this error on Arch.
I got the change to run my code on a Windows 7 machine, and didn't see
the problem there either. This seems to be specific to Haskell on Mac
OS X.
-- 
Jesper S?rnesj?
http://jesper.sarnesjo.org/
[1] https://github.com/bsl/GLFW-b/tree/master/glfw
------------------------------
Message: 3
Date: Mon, 11 Mar 2013 08:23:01 +1100
From: Jesper S?rnesj? 
Subject: Re: [Haskell-beginners] Performance problem with
   Haskell/OpenGL/GLFW
To: The Haskell-Beginners Mailing List - Discussion of primarily
   beginner-level topics related to Haskell 
Message-ID:
   
Content-Type: text/plain; charset=ISO-8859-1
On Mon, Mar 11, 2013 at 7:17 AM, Hollister Herhold  wrote:
...
Okay, I think I just figured this out. Well, HOW to get it working with the
accelerated renderer.
I was wondering a simple way to check renderer info so I ran glxinfo. This
(automatically) fired up X11, and then on a hunch I re-ran Test2 with X11
running and got this:
hhmacbook:~/Development/haskell/OpenGL:57> ./Test2
hardware
(2,7,7)
(3,2,0)
hhmacbook:~/Development/haskell/OpenGL:58>
AH HA! I then quit X11 and re-ran Test2, and got this:
hhmacbook:~/Development/haskell/OpenGL:58> ./Test2
software
(2,7,7)
(3,2,0)
hhmacbook:~/Development/haskell/OpenGL:59>
SO- If you want the accelerated renderer, you need to have X11 running.
Now, I have no idea WHY this is the case, but there you go.
Hope this helps.
This lead me down an interesting path.
First, I should explain that my machine, like most newish Macs, has
two graphics cards. In my case, a discrete Nvidia GeForce GT 330M, and
an integrated Intel chip. The former is better, but the latter uses
less power, and the system is supposed to switch between them
automatically.
I used gfxCardStatus [1] to show which card was in use. When I ran
test2.c, the system briefly switched to the discrete card. However,
when I ran Test2.hs, the system kept using the integrated chip the
whole time. Presumably, the Intel chip lacks a hardware implementation
of OpenGL 3.2, which causes the system to fall back to a software
renderer. I then used gfxCardStatus to force the system to *always*
use the discrete card and - boom! - this time Test2.hs received a
hardware renderer!
So it seems that the problem is a) Mac OS X-specific, or possibly
specific to systems with multiple graphics cards, b) related to
triggering the *switch* to the better graphics card. I don't yet
understand why the C program triggers a switch, while the Haskell
program does not, but I'll keep investigating.
Thank you all very much for your help!
-- 
Jesper S?rnesj?
http://jesper.sarnesjo.org/
[1] http://gfx.io
------------------------------
Message: 4
Date: Sun, 10 Mar 2013 18:27:56 -0400
From: Hollister Herhold 
Subject: Re: [Haskell-beginners] Performance problem with
   Haskell/OpenGL/GLFW
To: The Haskell-Beginners Mailing List - Discussion of primarily
   beginner-level topics related to Haskell 
Message-ID: <24E1A52D-CB7B-49A0-A0EC-9A932DBEDF73@fafoh.com>
Content-Type: text/plain;    charset=utf-8
I guess running X11 forces use of the NVidia chip. Interesting.
-Hollister
On Mar 10, 2013, at 5:23 PM, Jesper S?rnesj?  wrote:
...
On Mon, Mar 11, 2013 at 7:17 AM, Hollister Herhold  wrote:
...
Okay, I think I just figured this out. Well, HOW to get it working with the
accelerated renderer.
I was wondering a simple way to check renderer info so I ran glxinfo. This
(automatically) fired up X11, and then on a hunch I re-ran Test2 with X11
running and got this:
hhmacbook:~/Development/haskell/OpenGL:57> ./Test2
hardware
(2,7,7)
(3,2,0)
hhmacbook:~/Development/haskell/OpenGL:58>
AH HA! I then quit X11 and re-ran Test2, and got this:
hhmacbook:~/Development/haskell/OpenGL:58> ./Test2
software
(2,7,7)
(3,2,0)
hhmacbook:~/Development/haskell/OpenGL:59>
SO- If you want the accelerated renderer, you need to have X11 running.
Now, I have no idea WHY this is the case, but there you go.
Hope this helps.
This lead me down an interesting path.
First, I should explain that my machine, like most newish Macs, has
two graphics cards. In my case, a discrete Nvidia GeForce GT 330M, and
an integrated Intel chip. The former is better, but the latter uses
less power, and the system is supposed to switch between them
automatically.
I used gfxCardStatus [1] to show which card was in use. When I ran
test2.c, the system briefly switched to the discrete card. However,
when I ran Test2.hs, the system kept using the integrated chip the
whole time. Presumably, the Intel chip lacks a hardware implementation
of OpenGL 3.2, which causes the system to fall back to a software
renderer. I then used gfxCardStatus to force the system to *always*
use the discrete card and - boom! - this time Test2.hs received a
hardware renderer!
So it seems that the problem is a) Mac OS X-specific, or possibly
specific to systems with multiple graphics cards, b) related to
triggering the *switch* to the better graphics card. I don't yet
understand why the C program triggers a switch, while the Haskell
program does not, but I'll keep investigating.
Thank you all very much for your help!
-- 
Jesper S?rnesj?
http://jesper.sarnesjo.org/
[1] http://gfx.io
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://www.haskell.org/mailman/listinfo/beginners
------------------------------
Message: 5
Date: Sun, 10 Mar 2013 22:40:04 +0000
From: emacstheviking 
Subject: [Haskell-beginners] Trouble in (Fast)CGI land: matching `CGIT
   IO    a0' with actual type `IO ()'
To: The Haskell-Beginners Mailing List - Discussion of primarily
   beginner-level topics related to Haskell 
Message-ID:
   
Content-Type: text/plain; charset="utf-8"
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.

Francisco Gutiérrez

tags

participants (1)