plugin

A WebAssembly plugin.

Typst is capable of interfacing with plugins compiled to WebAssembly. Plugin functions may accept multiple byte buffers as arguments and return a single byte buffer. They should typically be wrapped in idiomatic Typst functions that perform the necessary conversions between native Typst types and bytes.

Plugins run in isolation from your system, which means that printing, reading files, or anything like that will not be supported for security reasons. To run as a plugin, a program needs to be compiled to a 32-bit shared WebAssembly library. Many compilers will use the WASI ABI by default or as their only option (e.g. emscripten), which allows printing, reading files, etc. This ABI will not directly work with Typst. You will either need to compile to a different target or stub all functions.

Plugins and Packages

Plugins are distributed as packages. A package can make use of a plugin simply by including a WebAssembly file and loading it. Because the byte-based plugin interface is quite low-level, plugins are typically exposed through wrapper functions, that also live in the same package.

Purity

Plugin functions must be pure: Given the same arguments, they must always return the same value. The reason for this is that Typst functions must be pure (which is quite fundamental to the language design) and, since Typst function can call plugin functions, this requirement is inherited. In particular, if a plugin function is called twice with the same arguments, Typst might cache the results and call your function only once.

Example

#let myplugin = plugin("hello.wasm")
#let concat(a, b) = str(
  myplugin.concatenate(
    bytes(a),
    bytes(b),
  )
)

#concat("hello", "world")

Protocol

To be used as a plugin, a WebAssembly module must conform to the following protocol:

Exports

A plugin module can export functions to make them callable from Typst. To conform to the protocol, an exported function should:

Take n 32-bit integer arguments a_1, a_2, ..., a_n (interpreted as lengths, so usize/size_t may be preferable), and return one 32-bit integer.
The function should first allocate a buffer buf of length a_1 + a_2 + ... + a_n, and then call wasm_minimal_protocol_write_args_to_buffer(buf.ptr).
The a_1 first bytes of the buffer now constitute the first argument, the a_2 next bytes the second argument, and so on.
The function can now do its job with the arguments and produce an output buffer. Before returning, it should call wasm_minimal_protocol_send_result_to_host to send its result back to the host.
To signal success, the function should return 0.
To signal an error, the function should return 1. The written buffer is then interpreted as an UTF-8 encoded error message.

Imports

Plugin modules need to import two functions that are provided by the runtime. (Types and functions are described using WAT syntax.)

(import "typst_env" "wasm_minimal_protocol_write_args_to_buffer" (func (param i32)))

Writes the arguments for the current function into a plugin-allocated buffer. When a plugin function is called, it receives the lengths of its input buffers as arguments. It should then allocate a buffer whose capacity is at least the sum of these lengths. It should then call this function with a ptr to the buffer to fill it with the arguments, one after another.
(import "typst_env" "wasm_minimal_protocol_send_result_to_host" (func (param i32 i32)))

Sends the output of the current function to the host (Typst). The first parameter shall be a pointer to a buffer (ptr), while the second is the length of that buffer (len). The memory pointed at by ptr can be freed immediately after this function returns. If the message should be interpreted as an error message, it should be encoded as UTF-8.