[Haskell-beginners] System.USB.writeInterrupt -- confused by error message from type system

emacstheviking objitsu at gmail.com
Wed Feb 27 12:46:47 CET 2013


I am building a USB gadget with a HexWax ExpandIO-USB chip, here:

The device is working fine (lsusb shows it) and so I thought that in the
name of continued training and doing things the hard way, that I would use
Haskell to talk to it!
I have only ever done USB with 'C' in the past and I am now stuck fighting
the type checker.

Here is my full code so far, it works in so far as it finds the device,
opens and closes a connection and prints out some stuff. I then wanted to
send four NUL bytes which elicits the same as a response according to the
documentation but I cannot get it to compile. Apologies for it in advance,
it's the journey you know, not the arrival, anyway, here it is...

module Main where

import Data.ByteString.Char8 as BS hiding (putStrLn)
import Data.Word (Word16)
import Data.Vector as V (filterM, null, (!)) --as V hiding
import System.USB
import System.Environment

-- hexwax expandIO-USB default code:
main :: IO ()
main = do
  findHexWax (0xb40,0x132) >>= \hw ->
    case hw of
      Just dev -> hexwaxGo dev
      Nothing  -> putStrLn "HexWax device not found"

findHexWax :: (Word16,Word16) -> IO (Maybe Device)
findHexWax (cVendor, cProd) = do
  usbCtx  <- newCtx
  usbDevs <- getDevices usbCtx
  setDebug usbCtx PrintWarnings
  boards <- V.filterM (isTarget (cVendor, cProd)) usbDevs
  case V.null boards of
    True  -> return Nothing
    False -> return $ Just $ boards!0
    isTarget :: (Word16, Word16) -> Device -> IO Bool
    isTarget (cVendor, cProd) dev = do
      info <- getDeviceDesc dev
      let vid = deviceVendorId info
      let vpr = deviceProductId info
      return $ ((vid, vpr) == (cVendor, cProd))

hexwaxGo :: Device -> IO ()
hexwaxGo dev = withDeviceHandle dev (testBoard dev)

testBoard :: Device -> DeviceHandle -> IO ()
testBoard dev handle = do
  putStrLn $ "Inspecting device: \"" ++ (show dev) ++ "\"\n"
  -- write 0x00 0x00 0x00 0x00, get back
  let payload  = pack "\x00\x00\x00\x00"
  let endPoint = EndpointAddress 0 Out
  action <- writeInterrupt handle endPoint
  (size, status) <- action payload 1000
  return ()

And the error message:

Prelude> :r
[1 of 1] Compiling Main             ( usb1.hs, interpreted )

    Couldn't match expected type `IO t0' with actual type `WriteAction'
    In the return type of a call of `writeInterrupt'
    Probable cause: `writeInterrupt' is applied to too few arguments
    In a stmt of a 'do' block: action <- writeInterrupt handle endPoint
    In the expression:
      do { putStrLn $ "Inspecting device: \"" ++ (show dev) ++ "\"";
           let payload = pack "\NUL\NUL\NUL\NUL";
           let endPoint = EndpointAddress 0 Out;
           action <- writeInterrupt handle endPoint;
           .... }
Failed, modules loaded: none.

I have spent a few hours trying this and that, my Haskell is improving but
not much good if I can't compile my program now is it!

So, can anybody point out the error of my ways here please as I just can't
see it!

Sean Charles.

PS: Code style comments, alternative ways of making it more concise (but
still readable and understandable mind you) are also welcome.
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