Nimterop is a Nim package that aims to make C/C++ interop seamless

Nim has one of the best FFI you can find - importing C/C++ is supported out of the box. All you need to provide is type and proc definitions for Nim to interop with C/C++ binaries. Generation of these wrappers is easy for simple libraries but quickly gets out of hand. c2nim greatly helps here by parsing and converting C/C++ into Nim but is limited due to the complex and constantly evolving C/C++ grammar. nimgen mainly focuses on automating the wrapping process and fills some holes but is again limited to c2nim's capabilities.

The goal of nimterop is to leverage the tree-sitter engine to parse C/C++ code and then convert relevant portions of the AST into Nim definitions using compile-time macros. tree-sitter is a Github sponsored project that can parse a variety of languages into an AST which is then leveraged by the Atom editor for syntax highlighting and code folding. The advantages of this approach are multifold:

• Benefit from the tree-sitter community's investment into language parsing
• Leverage Nim macros which are a user API and relatively stable
• Avoid depending on Nim compiler API which is evolving constantly

The nimterop feature set is still limited when compared with c2nim. Supported language constructs include:

• #define NAME VALUE where VALUE is a number (int, float, hex)
• struct X, typedef struct, enum X, typedef enum
• Functions with primitive types, structs, enums and typedef structs/enums as params and return values

Given the simplicity and success of this approach so far, it seems feasible to continue on for more complex code. The goal is to make interop seamless so nimterop will focus on wrapping headers and not the outright conversion of C/C++ implementation.

C++ constructs are still TBD depending on the results of the C interop.

Installation

Nimterop can be installed via Nimble:

nimble install "http://github.com/genotrance/nimtreesitter?subdir=treesitter"
nimble install "http://github.com/genotrance/nimtreesitter?subdir=treesitter_c"
nimble install "http://github.com/genotrance/nimtreesitter?subdir=treesitter_cpp"

nimble install http://github.com/genotrance/nimterop

or:

git clone http://github.com/genotrance/nimterop && cd nimterop
nimble installWithDeps

This will download and install nimterop in the standard Nimble package location, typically ~/.nimble. Once installed, it can be imported into any Nim program.

Usage

import nimterop/cimport

cDebug()
cDefine("HAS_ABC")
cDefine("HAS_ABC", "DEF")
cIncludeDir("clib/include")
cImport("clib.h")

cCompile("clib/src/*.c")

Refer to the tests directory for examples on how the library can be used.

Documentation

Detailed documentation is still forthcoming.

cDebug() - enable debug messages

cDefine("XXX") - #define an identifer that is forwarded to the C/C++ compiler using {.passC: "-DXXX".} as well as eventually used in processing #ifdef statements

cIncludeDir("XXX") - add an include directory that is forwarded to the compiler using {.passC: "-IXXX".}

cImport("header.h") - import all supported definitions from header file

cAddSearchDir("XXX") - add directory XXX to search path in calls to cSearchPath()

cAddStdDir("XXX") - add standard "c" [default] or "cpp" include paths to search path

cSearchPath("header.h") - return a file or directory found in search path configured using cSearchPath() - can be used in cCompile(), cIncludeDir() and cImport() calls

gitPull() - pull a git repository prior to C/C++ interop

Implementation Details

In order to use the tree-sitter C library at compile-time, it has to be compiled into a separate binary called toast (to AST) since the Nim VM doesn't yet support FFI. toast takes a C/C++ file and runs it through the tree-sitter API which returns an AST data structure. This is then printed out to stdout in a Lisp S-Expression format.

The cImport() proc runs toast on the specified header file and parses the resulting S-Expression back into an AST data structure at compile time. This AST is then processed to generate the relevant Nim definitions to interop with the code accordingly. A few other helper procs are provided to influence this process.

The tree-sitter library is limited as well - it may fail on some advanced language constructs but is designed to handle them gracefully since it is expected to have bad code while actively typing in an editor. When an error is detected, tree-sitter includes an ERROR node at that location in the AST. At this time, cImport() will complain and continue if it encounters any errors. Depending on how severe the errors are, compilation may succeed or fail. Glaring issues will be communicated to the tree-sitter team but their goals may not always align with those of this project.

Credits

Nimterop depends on tree-sitter and all licensing terms of tree-sitter apply to the usage of this package. Interestingly, the tree-sitter functionality is wrapped using c2nim and nimgen at this time. Depending on the success of this project, those could perhaps be bootstrapped using nimterop eventually.

Feedback

Nimterop is a work in progress and any feedback or suggestions are welcome. It is hosted on GitHub with an MIT license so issues, forks and PRs are most appreciated.