It is the first time I use ByteStrings and the first time I ever see a pcap file. I am basically trying to efficiently read the pcap file with ByteStrings and print its content on screen. I am using the Network.Pcap library to read the file. The ByteString variant can be found here: Network.Pcap ByteString. To make things easy I want just to print the first line of the file so my code looks like this:
1 import qualified Data.ByteString as B
2 printIt :: PktHdr -> B.ByteString -> IO ()
3 printIt ph bytep = do
4 print $ hdrCaptureLength ph -- not important
5 print $ bytep
6 main = do
7 f <- openOffline "file.pcap"
8 dispatchBS f (1) printIt
where printIt is the callbackBS function that operates on the body of the file.
The compiler complains with this message:
Couldn't match type ‘B.ByteString’
with ‘bytestring-0.10.4.0:Data.ByteString.Internal.ByteString’
NB: ‘B.ByteString’
is defined in ‘Data.ByteString.Internal’
in package ‘bytestring-0.10.4.1’
‘bytestring-0.10.4.0:Data.ByteString.Internal.ByteString’
is defined in ‘Data.ByteString.Internal’
in package ‘bytestring-0.10.4.0’
Expected type: CallbackBS
Actual type: PktHdr -> B.ByteString -> IO ()
In the third argument of ‘dispatchBS’, namely ‘printIt’
In a stmt of a 'do' block: dispatchBS f (1) printIt
What I understand is that for the compiler, the callbackBS function must have type: PktHdr -> ByteString -> IO (), while at line 2 the types is PktHdr ->B.ByteString -> IO ().
However I can not simply use the type ByteString because then I would incur in conflicts with functions defined in the prelude for normal lists.
Do you have any idea?
Compiler is trying to tell you that you are using two different bytestring packages. For the details and a solution see here: "Couldn't match expected type with actual type" error when using Codec.BMP