This module is an executable tutorial for the proto3-wire library. It will demonstrate how to encode and decode messages of various types.

Imports

We recommend importing the Proto3.Wire.Encode and Proto3.Wire.Decode modules qualified, since they define encoding and decoding functions with the same names.

The Proto3.Wire module reexports some useful functions, so a good default set of imports is:

import           Proto3.Wire
import qualified Proto3.Wire.Encode as Encode
import qualified Proto3.Wire.Decode as Decode

Primitives

Let's translate this simple .proto file into a Haskell data type and a pair of encoding and decoding functions:

message EchoRequest {
  string message = 1;
}

We begin by defining a data type to represent our messages:

data EchoRequest = EchoRequest { echoRequestMessage :: Text }

Encoding

To encode an EchoRequest, we use the Encode.text function, and provide the field number and the text value:

encodeEchoRequest :: EchoRequest -> Encode.MessageBuilder
encodeEchoRequest EchoRequest{..} =
    Encode.text 1 echoRequestMessage

Fields of type string can be encoded/decoded from/to values of type String, ByteString and Text. Here we use the Text type, which is encoded using the text function. Different primitive types have different encoding functions, which are usually named after the Protocol Buffers type.

Decoding

To decode an EchoRequest, we use the parse function, and provide a Parser to extract the fields:

decodeEchoRequest :: ByteString -> Either Decode.ParseError EchoRequest
decodeEchoRequest = Decode.parse echoRequestParser

The decoding function for Text is called Decode.text. However, we must specify the field number, which is done using the at function, and provide a default value, using the one function. The types will ensure that the field number and default value are provided.

We use the Functor instance for Parser to apply the EchoRequest constructor to the result:

echoRequestParser :: Decode.Parser Decode.RawMessage EchoRequest
echoRequestParser = EchoRequest <$> (one Decode.text mempty `at` 1

Messages with multiple fields

Let's make our example more interesting by including multiple fields:

message EchoResponse {
  string message = 1;
  uint64 timestamp = 2;
}

We begin by defining a data type to represent our messages:

data EchoResponse = EchoResponse { echoResponseMessage   :: Text
                                 , echoResponseTimestamp :: Word64
                                 }

Encoding

To encode messages with multiple fields, note that functions in the Proto3.Wire.Encode module return values in the MessageBuilder monoid, so we can use mappend to combine messages:

encodedEchoResponse :: EchoResponse -> Encode.MessageBuilder
encodedEchoResponse EchoResponse{..} =
    Encode.text 1 echoResponseMessage <>
        Encode.uint64 2 echoResponseTimestamp

However, be careful to always use increasing field numbers, since this is not enforced by the library.

Decoding

Messages with many fields can be parsed using the Applicative instance for Parser:

decodeEchoResponse :: ByteString -> Either Decode.ParseError EchoResponse
decodeEchoResponse = Decode.parse echoResponseParser

echoResponseParser :: Decode.Parser Decode.RawMessage EchoResponse
echoResponseParser = EchoResponse <$> (one Decode.text mempty `at` 1)
                                  <*> (one Decode.uint64 0 `at` 2)

Repeated Fields and Embedded Messages

Messages can be embedded in fields of other messages. This can be useful when entire sections of a message can be repeated or omitted.

Consider the following message types:

message EchoManyRequest {
  repeated EchoRequest requests = 1;
}

Again, we define a type corresponding to our message:

data EchoManyRequest = EchoManyRequest { echoManyRequestRequests :: Seq EchoRequest }

Encoding

Messages can be embedded using embedded.

In protocol buffers version 3, all fields are optional. To omit a value for a field, simply do not append it to the MessageBuilder.

Similarly, repeated fields can be encoded by concatenating several values with the same FieldNumber.

It can be useful to use foldMap to deal with these cases.

encodeEchoManyRequest :: EchoManyRequest -> Encode.MessageBuilder
encodeEchoManyRequest =
  foldMap (Encode.embedded 1 . encodeEchoRequest)
  . echoManyRequestRequests

Decoding

Embedded messages can be decoded using embedded.

Repeated fields can be decoded using repeated.

Repeated embedded messages can be decoded using repeated . Decode.embedded'.

decodeEchoManyRequest :: ByteString -> Either Decode.ParseError EchoManyRequest
decodeEchoManyRequest = Decode.parse echoManyRequestParser

echoManyRequestParser :: Decode.Parser Decode.RawMessage EchoManyRequest
echoManyRequestParser =
  EchoManyRequest <$> (repeated (Decode.embedded' echoRequestParser) `at` 1)