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:
-
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.
-
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?
- Install Haskell Stack Tool
- 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.
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:
- The Wire instances for the basic data types
- The API for the generated module to use to define their own Wire
instances
- 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".