a classy little scripting language

Embedding API

Wren is designed to be a scripting language, so the embedding API is as important as any of its language features. There are two (well, three) ways to get Wren into your application:

  1. Link to static or dynamic library. When you build Wren, it generates both shared and static libraries in lib that you can link to.

  2. Include the source directly in your application. If you want to include the source directly in your program, you don’t need to run any build steps. Just add the source files in src/vm to your project. They should compile cleanly as C99 or C++98 or anything later.

In either case, you also want to add src/include to your include path so you can get to the public header for Wren:

#include "wren.h"

Creating a Wren VM #

Once you’ve integrated the code into your executable, you need to create a virtual machine. To do that, you first fill in a WrenConfiguration:

WrenConfiguration config; 

This gives you a basic configuration that has reasonable defaults for everything. If you don’t need to tweak stuff, you can leave it at that. If you do want to turn some knobs and dials, it exposes some fields you can set:

config.reallocateFn = ...;

The reallocateFn is a callback you can provide to control how Wren allocates and frees memory. If you leave that to the default, it uses malloc() and free().

config.loadModuleFn = ...; 
config.bindForeignMethodFn = ...; 
config.bindForeignClassFn = ...;

These three callbacks are how Wren talks back to your program. We’ll cover them in detail later.

config.initialHeapSize = ...; 
config.minHeapSize = ...; 
config.heapGrowthPercent = ...;

These let you tune how the garbage collector runs. You can tweak these if you want, but the defaults are usually fine.

With this ready, you can create the VM:

WrenVM* vm = wrenNewVM(&config);

This allocates memory for a new VM using the same reallocateFn you provided. The Wren C implementation has no global state, so every single bit of data Wren uses is bundled up inside a WrenVM. You can have multiple Wren VMs running independently from each other without any problems.

wrenNewVM() stores its own copy of the configuration, so after calling it, you can discard the WrenConfiguration struct you filled in. Now you have a live VM, waiting to run some code!

Executing Wren code #

You can tell the VM to execute a string of Wren source code like so:

WrenInterpretResult result = wrenInterpret(vm, "System.print(\"Hi!\")");

The first string is the chunk of code to execute—a series of one or more statements separated by newlines. Wren runs this code in a special “main” module. Each time you call this, the code is run in the same module. This way, top-level names defined in one call can be accessed in later ones.

When you call wrenInterpret(), Wren first compiles your source to bytecode. If an error occurs here, it returns immediately with WREN_RESULT_COMPILE_ERROR. Otherwise, Wren spins up a new fiber and executes the code in that. Your code can in turn spawn whatever other fibers it wants. It keeps running fibers until they all complete.

If a runtime error occurs (and another fiber doesn’t catch it), it will abort fibers all the way back to the main one and then return WREN_RESULT_RUNTIME_ERROR. Otherwise, when the last fiber successfully returns, it returns WREN_RESULT_SUCCESS.

Calling a C function from Wren #


Calling a Wren method from C #


Storing a reference to a Wren object in C #


Storing C data in a Wren object #


Shutting down a VM #

Once the party is over and you’re ready to end your relationship with a VM, you need to free any memory it allocated. You do that like so:


After calling that, you obviously cannot use the WrenVM* you passed to it again. It’s dead.

Note that Wren will yell at you if you still have any live WrenValue objects when you call this. This makes sure you haven’t lost track of any of them (which leaks memory) and you don’t try to use any of them after the VM has been freed.