March 2013 - Beginners

Writing a custom pop function for a stack data type
by doaltan 12 Mar '13

12 Mar '13

Hi I have such a stack data structure: datamystack =Empty |Elem Char mystack derivingShow I'm trying to get the head of the stack using this: pophead :: Stack -> Char pophead Empty = Empty pophead (Element x stack) = x And I'm getting this error for the last sentence of the function : Not in scope: data constructor `Stack' Can you tell me how to fix it? Thanks.

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OpenGLRaw Tutorial
by Michael Baker 11 Mar '13

11 Mar '13

I'm trying to follow this book on OpenGL: http://www.arcsynthesis.org/gltut/Basics/Tut01%20Following%20the%20Data.html I'm trying to follow the examples using the OpenGLRaw package as the OpenGL package doesn't map very neatly to any of the examples. For instance, it is not clear to me now to create and use the vertex array used in the example that I linked to. However, I'm struggling a bit because I haven't used Haskell's foreign interface before. Here is an attempt which is expected to draw a triangle, but instead draws nothing: http://hpaste.org/83837 Does anyone know of a tutorial for OpenGLRaw or the foreign interface that might help me understand how to marshall data around? It seems like many people turn to OpenGLRaw when they're learning OpenGL so that they can follow the tutorials. I imagine it would be useful to have a guide out there that covers how to actually use it.

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Re: [Haskell-beginners] Beginners Digest, Vol 57, Issue 15
by Patrick Lynch 11 Mar '13

11 Mar '13

Good morning, ...anyone working with YESOD? Good day ----- Original Message ----- From: <beginners-request(a)haskell.org> To: <beginners(a)haskell.org> Sent: Monday, March 11, 2013 7:00 AM Subject: Beginners Digest, Vol 57, Issue 15 > Send Beginners mailing list submissions to > beginners(a)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(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)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: Using stack inside a function without declaring it as > input (Krzysztof Skrz?tnicki) > 2. Re: Using stack inside a function without declaring it as > input (Emanuel Koczwara) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Mon, 11 Mar 2013 11:15:32 +0100 > From: Krzysztof Skrz?tnicki <gtener(a)gmail.com> > Subject: Re: [Haskell-beginners] Using stack inside a function without > declaring it as input > To: doaltan <doaltan(a)yahoo.co.uk>, The Haskell-Beginners Mailing List > - Discussion of primarily beginner-level topics related to Haskell > <beginners(a)haskell.org> > Message-ID: > <CAM7aEVHagkBOORNwmsU-gHYm5sJixXg3hAojTDjnyA_x72tvWQ(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > There are some things clearly missing in your description. I think you > need > to read your problem more carefully and perhaps consult some accompanying > materials too. > > Other than that I see no way one can sensibly answer your question without > more information. > > Best regards, > Krzysztof Skrz?tnicki > > > On Mon, Mar 11, 2013 at 10:36 AM, doaltan <doaltan(a)yahoo.co.uk> wrote: > >> Hi I have a function like this : >> myfunc :: [Char] -> [Char] >> It is supposed to work pretty much like this : >> >> 1. Take a string >> 2. Put some elements of this input string to output string and put >> others to stack. >> 3. Pop elements to that output string too. >> 4. Do 2 and 3 recursively until stack is empty. >> 5. Print the output string when stack is empty. >> >> >> I couldn't figure out where to define stack and output string. Can you >> help me with that? I'm new to Haskell so I can't think in Haskell's logic >> very well. >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> >> >

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Using stack inside a function without declaring it as input
by doaltan 11 Mar '13

11 Mar '13

Hi I have a function like this : myfunc :: [Char] -> [Char] It is supposed to work pretty much like this : 1. Take a string 2. Put some elements of this input string to output string and put others to stack. 3. Pop elements to that output string too. 4. Do 2 and 3 recursively until stack is empty. 5. Print the output string when stack is empty. I couldn't figure out where to define stack and output string. Can you help me with that? I'm new to Haskell so I can't think in Haskell's logic very well.

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Trouble in (Fast)CGI land: matching `CGIT IO a0' with actual type `IO ()'
by emacstheviking 10 Mar '13

10 Mar '13

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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Re: [Haskell-beginners] Beginners Digest, Vol 57, Issue 12
by Francisco Gutiérrez 10 Mar '13

10 Mar '13

Enviado desde mi iPhone El 10/03/2013, a las 18:00, beginners-request(a)haskell.org escribió: > Send Beginners mailing list submissions to > beginners(a)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(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)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 <ky3(a)atamo.com> > 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 <beginners(a)haskell.org> > Message-ID: > <CAPY+ZdRYVFG=z=aACXJQ7p4t39Cc_dx6c4M_zKq8uiBSxF1=7A(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > On Mon, Mar 11, 2013 at 1:33 AM, Kim-Ee Yeoh <ky3(a)atamo.com> 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? <sarnesjo(a)gmail.com> > 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 <beginners(a)haskell.org> > Message-ID: > <CALex+WiznQHRYvAtOAeENcMOecnfmYg56+h2AK0uvdKD+L-YBg(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > On Mon, Mar 11, 2013 at 1:31 AM, Andrey Yankin <yankin013(a)gmail.com> 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? <sarnesjo(a)gmail.com> > 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 <beginners(a)haskell.org> > Message-ID: > <CALex+Wh2n6=92v1wnGpuxzXJU855PGLWAw3RrxAu+nLrCRq1zw(a)mail.gmail.com> > Content-Type: text/plain; charset=ISO-8859-1 > > On Mon, Mar 11, 2013 at 7:17 AM, Hollister Herhold <hollister(a)fafoh.com> 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 <hollister(a)fafoh.com> > 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 <beginners(a)haskell.org> > Message-ID: <24E1A52D-CB7B-49A0-A0EC-9A932DBEDF73(a)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? <sarnesjo(a)gmail.com> wrote: > >> On Mon, Mar 11, 2013 at 7:17 AM, Hollister Herhold <hollister(a)fafoh.com> 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(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners > > > > ------------------------------ > > Message: 5 > Date: Sun, 10 Mar 2013 22:40:04 +0000 > From: emacstheviking <objitsu(a)gmail.com> > 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 <beginners(a)haskell.org> > Message-ID: > <CAEiEuUKEZkEvyZSdWPCeT-fN94-n8OPtOdoUKnHgkwKihfZ3GA(a)mail.gmail.com> > 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. >

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Create new value for given type
by Heinrich Ody 10 Mar '13

10 Mar '13

Hi, I'm given a type 'a' (lets assume the type is infinite) and a set finite 'xs'. Now I want to create a new value of type 'a' that does not occur in 'xs' and return a set 'ys' that consists of 'xs' and also contains the new value. How can I do this??? The types are known at compile time, so I would think it is possible in Haskell.. Greetings, Heinrich

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Re: [Haskell-beginners] [Haskell] Newbie
by Brandon Allbery 07 Mar '13

07 Mar '13

On Thu, Mar 7, 2013 at 7:45 PM, Dan Lior <sitiposit(a)gmail.com> wrote: > 1) Is this the right place for newbies to post questions about Haskell? > This is most a list for announcements; beginners(a)haskell.org is better for these kinds of questions, and haskell-cafe(a)haskell.org for general discussion. pred :: Int -> Int > pred 0 = 0 > pred n+1 = n > n+k patterns were part of Haskell '98, but removed from Haskell 2010. You may be able to use the pragma {-# LANGUAGE NPlusKPatterns #-} to turn them back on. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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Converting a function to -> Writer w a
by Franco 07 Mar '13

07 Mar '13

I am trying to get comfortble with the various monads, just to hone my tools for a better tomorrow. > import Control.Monad.Writer > import Data.Char ( toLower ) > import Text.Regex.Posix ( (=~) ) So I decided to code a very small example to 'get into it'. The following code is extremely simple. Given a string, it will find all occourences of a specific regexp, replace them, and return the changed string back. > replaceStuff :: String -> String > replaceStuff "" = "" > replaceStuff cs = let (pre, found, post) = cs =~ pattern in > pre ++ transf found ++ replaceStuff post > where pattern = "[A-E]" -- some regExp pattern > transf = map toLower -- placeholder function > main :: IO () > main = putStrLn $ replaceStuff "this is thE Story of peter" Yay, it works! Now let's talk about the Writer monad. I would like to have a mini log via the writer monad (could be anything , probably "x found and replaced with y") > type Log = String But after that I have no idea how to add the writer monad to my simple but recursive function. Type sig will be something like replaceCols :: String -> Writer Log String but I don't know how to replace this part of the non monadic code pre ++ transf found ++ replaceStuff post I wrote this before giving up writer ((pre ++ found ++ ??whathere??), (found ++ " found\n")) >> Probably I am no "getting it", so any help is appreciated! Thanks -F

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Re: [Haskell-beginners] System.USB.writeInterrupt -- confused by error message from type system
by emacstheviking 06 Mar '13

06 Mar '13

Karol, Alexander, Thanks for your feedback... I am still a little confused as I shall explain... first of all let's look at the prototype for 'writeInterrupt', writeInterrupt :: DeviceHandle -> EndpointAddress -> WriteAction To me, that reads a "takes a device handle and an endpoint address and returns a WriteAction", and to quote the WriteAction help text verbatim so there is no confusion: type WriteAction = ByteString -> Timeout -> IO (Size, Status)Source Handy type synonym for write transfers. "A WriteAction is a function which takes a ByteString to write and a Timeout. The function returns an IO action which, when exectued(sic), returns the number of bytes that were actually written paired with a Status flag which indicates whether the transfer Completed or TimedOut." Now let's move to my original code and the 'right' code... action <- writeInterrupt handle endPoint let action = writeInterrupt handle endPoint If I understand things correctly up to this point, my mistake was being too eager in using "<-", my mind at that point was obviously confusing the return value from WriteAction with the return type of writeInterrupt and I can see now that what I should have done was use "let" which captures the WriteAction that is returned which can be executed with the payload and the timeout on the next line: (size, status) <- action payload 1000 On this line, the use of "<-" is what is required in order to cause the promised IO action to perform its duties and return me the tuple of data sent and status returned from the USB inner workings. However, we now come to the new type checker error, and this one has me floored right now. Time and time again I find myself beating my head against a wall and tearing my hair out trying to understand the thousand-and-one variations on strings in Haskell! I even tried the "string-conversions" (convertString) package but decided to battle it out instead... First the new code as edited in response to Karol: testBoard :: Device -> DeviceHandle -> IO () testBoard dev handle = do putStrLn $ "Inspecting device: \"" ++ (show dev) ++ "\"" -- write 0x00 0x00 0x00 0x00, get back same...we need to pad the -- packet out to 64 bytes for a full-speed device... should probably -- get this (64) from the device configuration / description record -- for maximum portability! let payload = BS.replicate 64 '\0' let endPoint = EndpointAddress 0 Out let action = writeInterrupt handle endPoint (size, status) <- action payload 1000 return () And the new error: usb1.hs:64:28: Couldn't match expected type `bytestring-0.9.2.1:Data.ByteString.Internal.ByteString' with actual type `ByteString' In the first argument of `action', namely `payload' In a stmt of a 'do' block: (size, status) <- action payload 1000 In the expression: do { putStrLn $ "Inspecting device: \"" ++ (show dev) ++ "\""; let payload = BS.replicate 64 '\NUL'; let endPoint = EndpointAddress 0 Out; let action = writeInterrupt handle endPoint; .... } Where and why does it think that "Data.ByteString.Internal.ByteString" is the type of the first parameter to "action" which is quite clearly stated as being "ByteString" ??? I know that "String", the native type is 4-bytes and that ByteString (Strict) and ByteString (Lazy) are both 8-bit, which is great, and I understand that the strict version (at least to me) feels like the rightmatch to be using for data buffers for a USB transfer but why oh why oh why can't I understand why the type checker picked up "internal" somewhere along the way? In the source code for WriteAction we have this: type WriteAction = B.ByteString → Timeout → IO (Size, Status) and at the top of the that source file: -- from bytestring: import qualified Data.ByteString as B ( ByteString, packCStringLen, drop, length ) import qualified Data.ByteString.Internal as BI ( createAndTrim, createAndTrim' ) import qualified Data.ByteString.Unsafe as BU ( unsafeUseAsCStringLen ) So why is it trying to be "internal"! I have tried not to be lazy, I have read everything and looked everywhere before posting again. If it had said: type WriteAction = BI.ByteString → Timeout → IO (Size, Status) I would have understood but it doesn't does it ?! Can somebody explain for me so I can just get on and write my killer USB application please! LOL :) Thanks, Sean. On 27 February 2013 12:07, Karol Samborski <edv.karol(a)gmail.com> wrote: > Hi Sean, > > I think that your function for testing board should look like this: > > testBoard :: Device -> DeviceHandle -> IO () > testBoard dev handle = do > putStrLn $ "Inspecting device: \"" ++ (show dev) ++ "\"\n" > -- write 0x00 0x00 0x00 0x00, get back same... > let payload = pack "\x00\x00\x00\x00" > let endPoint = EndpointAddress 0 Out > let action = writeInterrupt handle endPoint > (size, status) <- action payload 1000 > return () > > You need to use let because writeInterrupt returns (Timeout -> > ByteString -> IO (Size, Bool)) instead of IO (Timeout -> ByteString -> > IO (Size, Bool)) > > Karol > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

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