Google Protocol Buffers

Remarks#

To use Protocol Buffers with Haskell you should install the htprotoc package:

Clone the project from Github
Use Stack to build and install

You should now find the hprotoc executable in $HOME/.local/bin/.

Creating, building and using a simple .proto file

Let us first create a simple .proto file person.proto

package Protocol;

message Person {
    required string firstName = 1;
    required string lastName  = 2;
    optional int32  age       = 3;
}

After saving we can now create the Haskell files which we can use in our project by running

$HOME/.local/bin/hprotoc --proto_path=. --haskell_out=. person.proto

We should get an output similar to this:

Loading filepath: "/<path-to-project>/person.proto"
All proto files loaded
Haskell name mangling done
Recursive modules resolved
./Protocol/Person.hs
./Protocol.hs
Processing complete, have a nice day.

hprotoc will create a new folder Protocol in the current directory with Person.hs which we can simply import into our haskell project:

import Protocol (Person)

As a next step, if using Stack add

   protocol-buffers
 , protocol-buffers-descriptor

to build-depends: and

Protocol

to exposed-modules in your .cabal file.

If we get now a incoming message from a stream, the message will have the type ByteString.

In order to transform the ByteString (which obviously should contain encoded “Person” data) into our Haskell data type, we need to call the function messageGet which we import by

import Text.ProtocolBuffers (messageGet)

which enables to create a value of type Person using:

transformRawPerson :: ByteString -> Maybe Person
transformRawPerson raw = case messageGet raw of
    Left   _           -> Nothing
    Right (person, _)  -> Just person