[![Build Status](https://travis-ci.org/k-bx/protocol-buffers.svg)](https://travis-ci.org/k-bx/protocol-buffers) Haskell Protocol Buffers ==================================================== This the README file for `protocol-buffers`, `protocol-buffers-descriptors`, and `hprotoc`. These are three interdependent Haskell packages originally written by Chris Kuklewicz. Currently, maintainership was taken by Kostiantyn Rybnikov. It is planned to only support GHC 7.8 and newer unless someone explicitly asks for support of earlier versions. (Needs check) This README was updated most recently to reflect version `2.0.7`. This code should be compatible with Google protobuf version `2.3.0`. Changes to keep up with Google protobuf version `2.4.0` are being considered. What is this for? What does it do? Why? --------------------------------------- It is a pure Haskell re-implementation of the Google code at https://developers.google.com/protocol-buffers/docs/overview which is "...a language-neutral, platform-neutral, extensible way of serializing structured data for use in communications protocols, data storage, and more." Google's project produces C++, Java, and Python code. This one produces Haskell code. How well does this Haskell package duplicate Google's project? -------------------------------------------------------------- - This provides non-mutable messages that ought to be wire-compatible with Google. - These messages support extensions. - These messages support unknown fields if hprotoc is passed the proper flag (-u or --unknown_fields). - This does not generate anything for Services/Methods. - Adding support for services has not been considered. I think that Google's code checks for some policy violations that are not well documented enough for me to reverse engineer. Some (all?) of Google's APIs include the possibility of mutable messages. I suspect that my message reflection is not as useful at runtime as in some of Google's APIs. What is protocol-buffers? ------------------------- The protocol-buffers part is the main library which has two faces: 1. It provides an external API exported by module `Text.ProtocolBuffers` for users to read and write the binary format and manipulate the message data structures created by hprotoc. 2. It provides an internal API for the messages under module `Text.ProtocolBuffers.Header` to implement their tasks. What is protocol-buffers-descriptor? ------------------------------------ - It uses the `protocol-buffers` package. - It provides the code generated by hprotoc from `descriptor.proto` under module `Text.DescriptorProtos`. - This supports hprotoc which is used to describe proto files and the code they will generate. - It provides `Text.DescriptorProtos.Options` which help in looking up the new style custom options. What is hprotoc? ---------------- - It uses `protocol-buffers` and `protocol-buffers-descriptor` above. - It is a command line tool that reads in `.proto` files and produces Haskell source trees like Google's protoc. - ...and it contains a very nice lexer and parser for the `.proto` file... The hprotoc part is a executable program which reads `.proto` files and uses the `protocol-buffers` package to produce a tree of Haskell source files. The program is called `hprotoc`. Usage is given by the program itself, the options themselves are processed in order. It can take several input search paths, and allow an additional module prefix, a selectable output directory, and ends with a list of of proto file to generate from. The output has to be a tree of modules since each message is given its own namespace, and a module is the only partitioning of namespace in Haskell. The keys for extension fields are defined alongside the message whose namespace they share. Since message names are both a data type and a namespace the filename and the message name match (aside from the .hs file extension). And what are the examples and tests sub-directories? ---------------------------------------------------- The examples sub-directory is for duplicating the `addressbook.proto` example that Google has with its code. The `ABF` and `ABF2` file are included as binary addressbooks. These can be read by the C++ examples from Google, and vice-versa. The `tests` sub-directory is where I have written some test code to drive the `UnittestProto` code generated from Google's `unittest.proto` (and `unittest_import.proto`) files. The `patchBoot` file has the needed file patches to fix up the recursive imports (no longer needed!). What do I need to compile the code? ----------------------------------- 1. Install [Haskell Stack Tool](https://github.com/commercialhaskell/stack/) 2. Run `stack build` Alternatively, go with old-fashioned `cabal build`. How mature is this code? ------------------------ It can write the wire encoding and read it back. It has been tested for interoperability against Google's read/write code with `addressbook.proto`. `hprotoc` generates and uses the `Text.DescriptorProtos` tree from Google `descriptor.proto` file. `hprotoc` has generated code from `Google/protobuf/unittest.proto` and `Google/protobuf.unittest_import`. These compile after adding hs-boot files `TestAllExtensions.hs-boot`, `TestFieldOrderings.hs-boot`, and `TestMutualRecursionA.hs-boot` to resolve mutual recursion. The `TestEnumWithDupValue` has duplicated values which cause a compilation warning. There has been QuickCheck tests done for `UnittestProto/TestAllType.hs` and `UnittestProto/TestAllExtensions.hs` in the tests subdirectory. These pass as of `2008-09-19` for version `0.2.7`. These test that random messages can be roundtripped to the wire format without changing — with the caveat that the new extension keys are read back as raw bytes but compare equal because of the parsing done by (==). Mutual recursion is a problem? ------------------------------ Not using ghc. The haskell-src-exts let me generate code with `{-# SOURCE #-}` annotated imports. And `hprotoc` generates the needed hs-boot files for ghc. And key import cycles are broken by creating `Key.hs` files, which users can ignore. How stable is the API? ---------------------- This is the first working release of the code. I do not promise to keep any of the API but I am lazy so most things will not change. The reflection capabilities may get improved/altered. Stricter warnings and error detection may be added. Code will move between protocol-buffers and hprotoc projects. The internals of reading from the wire may be improved. Where is the API documentation? ------------------------------- Generate haddock with `stack haddock` command. You can also view API documentation online at [Hackage page](https://hackage.haskell.org/package/protocol-buffers). The imports of `Text.ProtocolBuffers` are the public API. The generated code's API is `Text.ProtocolBuffers.Header`. The only usage examples are in the `examples` sub-directory and the `tests` sub-directory. Since the messages are simply Haskell data types most of the manipulation should be easy. The main thing that is weird is that messages with extension ranges get an ExtField record field that holds ... an internal data structure. This is currently a `Map` from field number to a rather complicated existential + GADT combination that should really only be touched by the `ExtKey` and `MessageAPI` type class methods. The `ExtField` data constructor is not hidden, though it could be and probably ought to be. Note that extension fields are inherently slower, especially in ghci (though ghc's `-O2` helps quite a bit). The entire proto file is stored in the top level module in wire-encoded form and can be accessed as a `FileDescriptorProto`. The Haskell code also defines its own reflection data types, with one stored in each generated module and also in a master data type in the top level module (via `Show` and `Read`). Who reads this far? ------------------- I suspect no one ever will. Why define your own Haskell reflection types in addition to `FileDescriptorProto`s types? This allows for the protocol-buffers library package to not depend on a single thing defined in the `protocol-buffers-descriptor` package. This lack of recursion made for much simpler bootstrapping and allows the `descriptor.proto` generated files to be build separately. While `descriptor.proto` files are a great fit as output from parsing a proto file they are not as good a fit for code generation. They mix fields and extension keys, they have all optional fields even though some things (especially names) are compulsory. They obscure which descriptors are groups. They have a nested structure which is useful when resolving the names but not for iterating over for code generation. What are the pieces of protocol-buffers doing? ---------------------------------------------- - `Basic.hs` defines the core data types (that are not already in `Prelude`) and many classes. - `Mergeable.hs` defines the standard instances of `Mergeable` for combining types. - `Default.hs` defines the standard default of the basic data types. - `Reflections.hs` defines the Haskell reflection data types (stored with each generated module). - `Get.hs` is here because I needed a slightly different style of binary `Get` monad (see binary and binary-strict packages). This is standalone and could be put into any project. It has long comments inside. - `WireMessage.hs` defines 3 things: 1. The Wire instances for the basic data types 2. The API for the generated module to use to define their own Wire instances 3. The API for the user to load and save messages This file would not compile with ghc-6.8.3 on a G4 (Mac OS X 10.5.4, XCode 3.1) without -fvia-C as the cabal file states. - `Extensions.hs` is rather large because it add everything needed for extension fields (see haddock API docs). It should not export ExtField's constructor, but it currently does. - `Header.hs` re-exports what is needed for the instance messages. - `ProtocolBuffer.hs` re-exports what is needed for the user API. What are the pieces of hprotoc doing? ------------------------------------- `alex` uses `Lexer.x` to generated `Lexer.hs` which slices up the `.proto` file into tokens. The `.proto` layout is well designed, quite unambiguous, and easy to tokenize. The lexer also does the jobs of decoding the backslash escape codes in quotes strings, and interpreting floating point numbers. Errors and unexpected input are inserted into the token list, with at least line number level precision. The `Parser.hs` file has a `Parsec` parser which are really used as nested parsers (allowing for the type of the user state to change). The `.proto` grammar is well designed and the system never needs to backtrack over tokens. The default values and options' values parsed according to the expected type, and string default are check for valid utf8 encoding. (This also import the `Instances.hs` file) The `Resolve.hs` has code to resolve all the names to a fully qualified form, including name mangling where necessary. This includes code to load and parse all the imported `.proto` files, reusing parses for efficiency, and detecting import loops. The context built from each imported file is combined to change the `FileDescriptorProto` into a modified `FileDescriptorProto`. This stage also determines that extension keys are in a valid extensions range declaration, and enum default values exists. The `MakeReflections.hs` file converts the nested `FileDescriptorProto` into a flatter Haskell reflection data structure. This includes parsing the default value stored in the `FileDescriptorProto`. The `BreakRecursion.hs` file builds graphs describing the imports and works out whether and how to create hs-boot and `Key.hs` files to allow allow for warning-free compilation with ghc (as of 6.10.1). The `Gen.hs` file takes a Haskell data structure from `MakeReflections` and builds a module syntax data structure. The syntax data is quite verbose and several helper functions are used to help with the composition. The result is easy to print as a string to a file. The `ProtoCompile.hs` file is the Main module which defines the command line program `hprotoc`. This manages most of the interaction with the file system (aside from import loading in Resolve). Everything that is needed is collected into the Options data type which is passed to "run". The output style can be tweaked by changing "style" and "myMode". New oneof implementation ------------------------ Since `protocol-buffers` version 2.6, the upstream `protocol-buffers` supports [oneof](https://developers.google.com/protocol-buffers/docs/proto?hl=en#oneof) keyword, which is a union of different data types. It is very natural to combine the oneof specification into Haskell ADT, so we implement the feature. In `hprotoc/oneoftest`, we have an example for this. `school.proto` defines a collection of members in a school, which is organized into dormitories. Each member should have common attributes like `id` and `name`, but there are attributes only specific to students, faculties or administrators. Therefore, we define property as a oneof field which is one of student, faculty and admin type. How it is defined should be easily understood from `school.proto`. Once `protocol-buffers` is installed, using `hprotoc`, we can generate Haskell source codes. Assuming we run `hprotoc` on the `oneoftest` directory and generate source code in `hs` directory: ``` oneoftest> hprotoc --proto_path=. --haskell_out=hs school.proto ``` We will have `School.Member` module which defines `Member` by (I omit qualifier and strictness annotation here.) ``` data Member = Member { id :: Int32 , name :: Utf8 , property :: Maybe Property } ``` and `School.Member.Property` module defines `Property` (as `oneof`) by ``` data Property = Prop_student {prop_student :: Student } | Prop_faculty {prop_faculty :: Faculty } | Prop_admin {prop_admin :: Admin } ``` where `Student`, `Faculty` and `Admin` are defined as ordinary nested message data types in separate modules, respectively. Therefore, the `oneof` feature is smoothly matched with Haskell sum types. Note that `Maybe` will be always present for a `oneof` field in the owner data type definition (Here, `Maybe Property` in the definition of `Member`). This is because of compatibility with other language implementations that treat `oneof` as a collection of `optional` fields. In the `oneoftest` directory, we provides a Haskell example in `hprotoc/oneoftest/hs`, modification of the previous example using lenses in `hprotoc/oneoftest/hs-lens` and a C++ example in `hprotoc/oneoftest/cpp` to demonstrate how to use. Each example has `encode` and `decode`. With `encode`, we start from data in memory and generate a serialized binary file in wire format. Then,`decode` takes the file and present some information to prove it successfully decoded the binary. One can encode from Haskell side and decode on C++ side, or vice versa. For building, simply run `build.sh` in each of `hs` or `cpp` directories. Example with lenses has a `stack.yml` in it so it could be easily built with `stack build`. For C++, one must previously install C++ `protobuf` library and `pkg-config`. We assume that `hprotoc` was already executed as shown above. The version with lenses requires slightly different command: ``` oneoftest> hprotoc --proto_path=. --haskell_out=hs-lens/src school.proto ``` Here are examples. ``` hs > ./encode serialized.dat hs > cd ../cpp cpp> ./decode ../hs/serialized.dat name: "Gryffindor" members { id: 1 name: "Albus Dumbledore" prop_faculty { subject: "allmighty" title: "headmaster" } } members { id: 2 name: "Harry Potter" prop_student { grade: 5 specialty: "defense of dark arts" } } ``` ``` cpp> ./encode serialized.dat cpp> cd ../hs hs > ./decode ../cpp/serialized.dat Right (Dormitory {name = "Gryffindor", members = fromList [Member {id = 1, name = "Albus Dumbledore", property = Just (Prop_faculty {prop_faculty = Faculty {subject = "allmighty", title = Just "headmaster", duty = fromList []}})},Member {id = 2, name = "Harry Potter", property = Just (Prop_student {prop_student = Student {grade = 5, specialty = Just "defense of dark arts"}})}]},"") ```