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128 lines
7.8 KiB
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---
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layout: page
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title: Other Languages
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---
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# Other Languages
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Cap'n Proto's reference implementation is in C++. Implementations in other languages are
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maintained by respective authors and have not been reviewed by me
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([@kentonv](https://github.com/kentonv)). Below are the implementations I'm aware
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of. Some of these projects are more "ready" than others; please consult each
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project's documentation for details.
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##### Serialization + RPC
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* [C++](cxx.html) by [@kentonv](https://github.com/kentonv)
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* [C# (.NET Core)](https://github.com/c80k/capnproto-dotnetcore) by [@c80k](https://github.com/c80k)
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* [Erlang](http://ecapnp.astekk.se/) by [@kaos](https://github.com/kaos)
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* [Go](https://github.com/zombiezen/go-capnproto2) by [@zombiezen](https://github.com/zombiezen) (forked from [@glycerine](https://github.com/glycerine)'s serialization-only version, below)
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* [Haskell](https://github.com/zenhack/haskell-capnp) by [@zenhack](https://github.com/zenhack)
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* [JavaScript (Node.js only)](https://github.com/capnproto/node-capnp) by [@kentonv](https://github.com/kentonv)
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* [OCaml](https://github.com/capnproto/capnp-ocaml) by [@pelzlpj](https://github.com/pelzlpj) with [RPC](https://github.com/mirage/capnp-rpc) by [@talex5](https://github.com/talex5)
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* [Python](http://capnproto.github.io/pycapnp/) by [@jparyani](https://github.com/jparyani)
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* [Rust](https://github.com/dwrensha/capnproto-rust) by [@dwrensha](https://github.com/dwrensha)
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##### Serialization only
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* [C](https://github.com/opensourcerouting/c-capnproto) by [OpenSourceRouting](https://www.opensourcerouting.org/) / [@eqvinox](https://github.com/eqvinox) (originally by [@jmckaskill](https://github.com/jmckaskill))
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* [D](https://github.com/capnproto/capnproto-dlang) by [@ThomasBrixLarsen](https://github.com/ThomasBrixLarsen)
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* [Go](https://github.com/glycerine/go-capnproto) by [@glycerine](https://github.com/glycerine) (originally by [@jmckaskill](https://github.com/jmckaskill))
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* [Java](https://github.com/capnproto/capnproto-java/) by [@dwrensha](https://github.com/dwrensha)
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* [JavaScript](https://github.com/capnp-js/plugin/) by [@popham](https://github.com/popham)
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* [JavaScript](https://github.com/jscheid/capnproto-js) (older, abandoned) by [@jscheid](https://github.com/jscheid)
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* [Lua](https://github.com/cloudflare/lua-capnproto) by [CloudFlare](http://www.cloudflare.com/) / [@calio](https://github.com/calio)
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* [Nim](https://github.com/zielmicha/capnp.nim) by [@zielmicha](https://github.com/zielmicha)
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* [Ruby](https://github.com/cstrahan/capnp-ruby) by [@cstrahan](https://github.com/cstrahan)
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* [Scala](https://github.com/katis/capnp-scala) by [@katis](https://github.com/katis)
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##### Tools
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These are other misc projects related to Cap'n Proto that are not actually implementations in
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new languages.
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* [Common Test Framework](https://github.com/kaos/capnp_test) by [@kaos](https://github.com/kaos)
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* [Sublime Syntax Highlighting](https://github.com/joshuawarner32/capnproto-sublime) by
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[@joshuawarner32](https://github.com/joshuawarner32)
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* [Vim Syntax Highlighting](https://github.com/cstrahan/vim-capnp) by [@cstrahan](https://github.com/cstrahan)
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* [Wireshark Dissector Plugin](https://github.com/kaos/wireshark-plugins) by [@kaos](https://github.com/kaos)
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* [VS Code Syntax Highlighter](https://marketplace.visualstudio.com/items?itemName=xmonader.vscode-capnp) by [@xmonader](https://github.com/xmonader)
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* [IntelliJ Syntax Highlighter](https://github.com/xmonader/sercapnp) by [@xmonader](https://github.com/xmonader)
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## Contribute Your Own!
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We'd like to support many more languages in the future!
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If you'd like to own the implementation of Cap'n Proto in some particular language,
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[let us know](https://groups.google.com/group/capnproto)!
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**You should e-mail the list _before_ you start hacking.** We don't bite, and we'll probably have
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useful tips that will save you time. :)
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**Do not implement your own schema parser.** The schema language is more complicated than it
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looks, and the algorithm to determine offsets of fields is subtle. If you reuse the official
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parser, you won't risk getting these wrong, and you won't have to spend time keeping your parser
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up-to-date. In fact, you can still write your code generator in any language you want, using
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compiler plugins!
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### How to Write Compiler Plugins
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The Cap'n Proto tool, `capnp`, does not actually know how to generate code. It only parses schemas,
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then hands the parse tree off to another binary -- known as a "plugin" -- which generates the code.
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Plugins are independent executables (written in any language) which read a description of the
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schema from standard input and then generate the necessary code. The description is itself a
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Cap'n Proto message, defined by
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[schema.capnp](https://github.com/sandstorm-io/capnproto/blob/master/c%2B%2B/src/capnp/schema.capnp).
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Specifically, the plugin receives a `CodeGeneratorRequest`, using
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[standard serialization](encoding.html#serialization-over-a-stream)
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(not packed). (Note that installing the C++ runtime causes schema.capnp to be placed in
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`$PREFIX/include/capnp` -- `/usr/local/include/capnp` by default).
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Of course, because the input to a plugin is itself in Cap'n Proto format, if you write your
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plugin directly in the language you wish to support, you may have a bootstrapping problem: you
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somehow need to generate code for `schema.capnp` before you write your code generator. Luckily,
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because of the simplicity of the Cap'n Proto format, it is generally not too hard to do this by
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hand. Remember that you can use `capnp compile -ocapnp schema.capnp` to get a dump of the sizes
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and offsets of all structs and fields defined in the file.
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`capnp compile` normally looks for plugins in `$PATH` with the name `capnpc-[language]`, e.g.
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`capnpc-c++` or `capnpc-capnp`. However, if the language name given on the command line contains
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a slash character, `capnp` assumes that it is an exact path to the plugin executable, and does not
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search `$PATH`. Examples:
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# Searches $PATH for executable "capnpc-mylang".
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capnp compile -o mylang addressbook.capnp
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# Uses plugin executable "myplugin" from the current directory.
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capnp compile -o ./myplugin addressbook.capnp
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If the user specifies an output directory, the compiler will run the plugin with that directory
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as the working directory, so you do not need to worry about this.
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For examples of plugins, take a look at
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[capnpc-capnp](https://github.com/sandstorm-io/capnproto/blob/master/c%2B%2B/src/capnp/compiler/capnpc-capnp.c%2B%2B)
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or [capnpc-c++](https://github.com/sandstorm-io/capnproto/blob/master/c%2B%2B/src/capnp/compiler/capnpc-c%2B%2B.c%2B%2B).
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### Supporting Dynamic Languages
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Dynamic languages have no compile step. This makes it difficult to work `capnp compile` into the
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workflow for such languages. Additionally, dynamic languages are often scripting languages that do
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not support pointer arithmetic or any reasonably-performant alternative.
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Fortunately, dynamic languages usually have facilities for calling native code. The best way to
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support Cap'n Proto in a dynamic language, then, is to wrap the C++ library, in particular the
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[C++ dynamic API](cxx.html#dynamic-reflection). This way you get reasonable performance while
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still avoiding the need to generate any code specific to each schema.
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To parse the schema files, use the `capnp::SchemaParser` class (defined in `capnp/schema-parser.h`).
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This way, schemas are loaded at the same time as all the rest of the program's code -- at startup.
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An advanced implementation might consider caching the compiled schemas in binary format, then
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loading the cached version using `capnp::SchemaLoader`, similar to the way e.g. Python caches
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compiled source files as `.pyc` bytecode, but that's up to you.
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### Testing Your Implementation
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The easiest way to test that you've implemented the spec correctly is to use the `capnp` tool
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to [encode](capnp-tool.html#encoding-messages) test inputs and
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[decode](capnp-tool.html#decoding-messages) outputs.
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