CODEWITHSUNDEEP
haskellpcapbytestring
mastro
mastro

Reputation: 619

Haskell ByteString type error

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?

Upvotes: 0

Views: 413

Answers (1)

Yuras
Yuras

Reputation: 13876

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

Upvotes: 2

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