Extism MoonBit PDK: This library can be used to write Extism Plug-ins in MoonBit.

extism

pdk

moonbit

moon add gmlewis/moonbit-pdk@0.47.28

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Author

gmlewis

Version

0.47.28

License

Apache-2.0

Repository

github.com/gmlewis/moonbit-pdk

Last updated

29 days ago

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README

#gmlewis/moonbit-pdk

This is an Extism PDK that can be used to write Extism Plug-ins using the MoonBit programming language.

#Install

Add the library to your project as a dependency with the moon tool:

moon add gmlewis/moonbit-pdk

#Reference Documentation

You can find the reference documentation for this library on mooncakes.io:

Examples can also be found there:

#Getting Started

The goal of writing an Extism plug-in is to compile your MoonBit code to a Wasm module with exported functions that the host application can invoke. The first thing you should understand is creating an export. Let's write a simple program that exports a greet function which will take a name as a string and return a greeting string.

First, install the moon CLI tool:

See https://www.moonbitlang.com/download/ for instructions for your platform.

Create a new MoonBit project directory using the moon tool and initialize the project:

moon new greet
cd greet

Next, add this Extism PDK to the project and remove the default "lib" example:

moon add gmlewis/moonbit-pdk
rm -rf lib

Now paste this into your main/main.mbt file:

pub fn greet() -> Int {
  let name = @host.input_string()
  let greeting = "Hello, \{name}!"
  @host.output_string(greeting)
  0 // success
}

fn main {

}

Then paste this into your main/moon.pkg.json file to export the greet function and include the @host import into your plugin:

{
  "import": [
    "gmlewis/moonbit-pdk/pdk/host"
  ],
  "link": {
    "wasm": {
      "exports": [
        "greet"
      ],
      "export-memory-name": "memory"
    }
  }
}

Some things to note about this code:

The moon.pkg.json file is required. This marks the greet function as an export with the name greet that can be called by the host.
We need a main but it is unused.
Exports in the MoonBit PDK are coded to the raw ABI. You get parameters from the host by calling @host.input* functions and you send return values back with the @host.output* functions.
An Extism export expects an i32 (a MoonBit Int) return code. 0 is success and 1 (or any other value) is a failure.

Finally, compile this with the command:

moon build --target wasm

We can now test plugin.wasm using the Extism CLI's run command:

extism call target/wasm/release/build/main/main.wasm greet --input "Benjamin" --wasi
# => Hello, Benjamin!

Note: We also have a web-based, plug-in tester called the Extism Playground

#More Exports: Error Handling

Suppose we want to re-write our greeting module to never greet Benjamins. We can use @host.set_error:

pub fn greet() -> Int {
  let name = @host.input_string()
  if name == "Benjamin" {
    @host.set_error("Sorry, we don't greet Benjamins!")
    return 1 // failure
  }
  let greeting = "Hello, \{name}!"
  @host.output_string(greeting)
  0 // success
}

Now when we try again:

moon build --target wasm
extism call target/wasm/release/build/main/main.wasm greet --input "Benjamin" --wasi
# => Error: Sorry, we don't greet Benjamins!
echo $? # print last status code
# => 1
extism call target/wasm/release/build/main/main.wasm greet --input "Zach" --wasi
# => Hello, Zach!
echo $?
# => 0

#JSON

Extism export functions simply take bytes in and bytes out. Those can be whatever you want them to be. A common way to get more complex types to and from the host is with JSON: (MoonBit currently requires a bit of boilerplate to handle JSON I/O but hopefully this situation will improve as the standard library is fleshed out.)

struct Add {
  a : Int
  b : Int
}

pub fn Add::from_json(value : Json) -> Add? {
  // From: https://github.com/moonbitlang/core/issues/892#issuecomment-2306068783
  match value {
    { "a": Number(a), "b": Number(b) } => Some({ a: a.to_int(), b: b.to_int() })
    _ => None
  }
}

type! ParseError String derive(Show)

pub fn Add::parse(s : String) -> Add!ParseError {
  match @json.parse?(s) {
    Ok(jv) =>
      match Add::from_json(jv) {
        Some(value) => value
        None => raise ParseError("unable to parse Add \{s}")
      }
    Err(e) => raise ParseError("unable to parse Add \{s}: \{e}")
  }
}

struct Sum {
  sum : Int
} derive(ToJson)

pub fn add() -> Int {
  let input = @host.input_string()
  let params = try {
    Add::parse!(input)
  } catch {
    ParseError(e) => {
      @host.set_error(e)
      return 1
    }
  }
  //
  let sum = { sum: params.a + params.b }
  let json_value = sum.to_json()
  @host.output_json_value(json_value)
  0 // success
}

Export your add function in main/moon.pkg.json:

{
  "import": [
    "gmlewis/moonbit-pdk/pdk/host"
  ],
  "link": {
    "wasm": {
      "exports": [
        "add"
      ],
      "export-memory-name": "memory"
    }
  }
}

Then compile and run:

moon build --target wasm
extism call plugin.wasm add --input='{"a": 20, "b": 21}' --wasi
# => {"sum":41}

#Configs

Configs are key-value pairs that can be passed in by the host when creating a plug-in. These can be useful to statically configure the plug-in with some data that exists across every function call.

Here is a trivial example using config.get:

pub fn greet() -> Int {
  let user = match @config.get("user") {
    Some(user) => user
    None => {
      @host.set_error("This plug-in requires a 'user' key in the config")
      return 1 // failure
    }
  }
  let greeting = "Hello, \{user}!"
  @host.output_string(greeting)
  0 // success
}

Remember to import the config and host packages in main/moon.pkg.json and export your function:

{
  "import": [
    "gmlewis/moonbit-pdk/pdk/config",
    "gmlewis/moonbit-pdk/pdk/host"
  ],
  "link": {
    "wasm": {
      "exports": [
        "greet"
      ],
      "export-memory-name": "memory"
    }
  }
}

To test it, the Extism CLI has a --config option that lets you pass in key=value pairs:

moon build --target wasm
extism call target/wasm/release/build/main/main.wasm greet --config user=Benjamin
# => Hello, Benjamin!
extism call target/wasm/release/build/main/main.wasm greet
# => Error: This plug-in requires a 'user' key in the config

#Variables

Variables are another key-value mechanism but are a mutable data store that will persist across function calls. These variables will persist as long as the host has loaded and not freed the plug-in.

pub fn count() -> Int {
  let mut count = match @var.get_int("count") {
    Some(v) => v
    None => 0
  }
  count = count + 1
  @var.set_int("count", count)
  let s = count.to_string()
  @host.output_string(s)
  0 // success
}

Note: Use the untyped variant @var.set_bytes to handle your own types.

Remember to import the host and var packages in main/moon.pkg.json and export your function:

{
  "import": [
    "gmlewis/moonbit-pdk/pdk/host",
    "gmlewis/moonbit-pdk/pdk/var"
  ],
  "link": {
    "wasm": {
      "exports": [
        "count"
      ],
      "export-memory-name": "memory"
    }
  }
}

#Logging

Because Wasm modules by default do not have access to the system, printing to stdout won't work (unless you use WASI). Extism provides simple logging functions that allow you to use the host application to log without having to give the plug-in permission to make syscalls.

pub fn log_stuff() -> Int {
  @host.log_info_str("An info log!")
  @host.log_debug_str("A debug log!")
  @host.log_warn_str("A warn log!")
  @host.log_error_str("An error log!")
  0 // success
}

From Extism CLI:

moon build --target wasm
extism call target/wasm/release/build/main/main.wasm log_stuff --wasi --log-level=trace
# => 2024/07/09 11:37:30 No runtime detected
# => 2024/07/09 11:37:30 Calling function : log_stuff
# => 2024/07/09 11:37:30 An info log!
# => 2024/07/09 11:37:30 A debug log!
# => 2024/07/09 11:37:30 A warn log!
# => 2024/07/09 11:37:30 An error log!

Note: From the CLI you need to pass a level with --log-level. If you are running the plug-in in your own host using one of our SDKs, you need to make sure that you call set_log_file to "stdout" or some file location.

#HTTP

Sometimes it is useful to let a plug-in make HTTP calls. See this example.

pub fn http_get() -> Int {
  // create an HTTP Request (without relying on WASI), set headers as needed
  let req = @http.new_request(
    @http.Method::GET,
    "https://jsonplaceholder.typicode.com/todos/1",
  )
  req.header.set("some-name", "some-value")
  req.header.set("another", "again")
  // send the request, get response back
  let res = req.send()

  // zero-copy send output to host
  res.output()
  0 // success
}

By default, Extism modules cannot make HTTP requests unless you specify which hosts it can connect to. You can use --alow-host in the Extism CLI to set this:

extism call \
    target/wasm/release/build/examples/http-get/http-get.wasm \
    http_get \
    --wasi \
    --allow-host='*.typicode.com'
# => {
# =>   "userId": 1,
# =>   "id": 1,
# =>   "title": "delectus aut autem",
# =>   "completed": false
# => }

#Imports (Host Functions)

Like any other code module, Wasm not only lets you export functions to the outside world, you can import them too. Host Functions allow a plug-in to import functions defined in the host. For example, if your host application is written in Python, it can pass a Python function down to your MoonBit plug-in where you can invoke it.

This topic can get fairly complicated and we have not yet fully abstracted the Wasm knowledge you need to do this correctly. So we recommend reading our concept doc on Host Functions before you get started.

#A Simple Example

Host functions have a similar interface as exports. You just need to declare them as external in your main.mbt. You only declare the interface as it is the host's responsibility to provide the implementation:

pub fn a_python_func(offset : Int64) -> Int64 = "extism:host/user" "a_python_func"

We should be able to call this function as a normal Go function. Note that we need to manually handle the pointer casting:

pub fn hello_from_python() -> Int {
  let msg = "An argument to send to Python"
  let mem = @host.allocate_string(msg)
  let ptr = a_python_func(mem.offset)
  mem.free()
  let rmem = @host.find_memory(ptr)
  let response = rmem.to_string()
  @host.output_string(response)
  return 0
}

#Testing it out

We can't really test this from the Extism CLI as something must provide the implementation. So let's write out the Python side here. Check out the docs for Host SDKs to implement a host function in a language of your choice.

from extism import host_fn, Plugin

@host_fn()
def a_python_func(input: str) -> str:
    # just printing this out to prove we're in Python land
    print("Hello from Python!")

    # let's just add "!" to the input string
    # but you could imagine here we could add some
    # applicaiton code like query or manipulate the database
    # or our application APIs
    return input + "!"

Now when we load the plug-in we pass the host function:

manifest = {"wasm": [{"path": "target/wasm/release/build/main/main.wasm"}]}
plugin = Plugin(manifest, functions=[a_python_func], wasi=True)
result = plugin.call('hello_from_python', b'').decode('utf-8')
print(result)

moon build --target wasm
python3 -m pip install extism
python3 app.py
# => Hello from Python!
# => An argument to send to Python!

Note: This fails on my Mac M2 Max with some weird system error but works great on my Linux Mint Cinnamon box.

#For PDK Devs: Building the PDK locally

Before building, you must have already installed the MoonBit programming language, the Go programming language, and the Extism CLI tool.

To install MoonBit, follow the instructions here (it is super-easy with VSCode): https://www.moonbitlang.com/download/

Then, to build this PDK, clone the repo, and type:

moon update && moon install
./build.sh

#Run

To run the examples, type:

./run.sh

#Status

The code has been updated to support compiler:

$ moon version --all
moon 0.1.20260713 (75c7e1f 2026-07-13) ~/.moon/bin/moon
moonc v0.10.4+2cc641edf (2026-07-15) ~/.moon/bin/moonc
moonrun 0.1.20260713 (75c7e1f 2026-07-13) ~/.moon/bin/moonrun

#Reach Out!

Have a question or just want to drop in and say hi? Hop on the Discord!

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