Haskell Language Google Protocol Buffers Creating, building and using a simple .proto file
Example
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
