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Using the OCaml Host SDK

Check your installation

Please be sure you've installed Extism before continuing with this guide.

1. Install the OCaml library

Extism is available on opam:

opam install extism

2. Import the library and use the APIs

Count Vowels Plugin

code.wasm in this example is our example plugin that counts vowels. If you want to run this, download it first and set the path:

curl > code.wasm
open Extism

let () =
let input =
if Array.length Sys.argv > 1 then Sys.argv.(1) else "this is a test"
let ctx = Context.create () in
let manifest = Manifest.(create [ Wasm.file "../wasm/code.wasm" ]) in
(* NOTE: if you encounter an error such as:
"Unable to load plugin: unknown import: wasi_snapshot_preview1::fd_write has not been defined"
use [Plugin.of_manifest ~wasi:true] in the following line to provide WASI imports to your plugin. *)
let plugin = Plugin.of_manifest ctx manifest |> Error.unwrap in
let res = plugin ~name:"count_vowels" input |> Error.unwrap in
print_endline res; ctx

Host Functions

It is also possible to create functions to expose additional functionality from the host. The first step is to define a function with the proper signature:

let hello_world =
let open Val_type in
Function.create "hello_world" ~params:[ I64 ] ~results:[ I64 ]
~user_data:"Hello again!"
@@ fun plugin params results user_data ->
let open Types.Val_array in

(* Print input argument *)
let mem = Current_plugin.Memory_block.of_val_exn plugin params.$[0] in
let s = Current_plugin.Memory_block.get_string plugin mem in
let () = print_endline s in

(* Print user data *)
let () = print_endline (string_of_int user_data) in

(* Set output pointer to input pointer *)
results.$[0] <- params.$[0]

Then add it to the plugin when it's created:

let functions = [ hello_world ] in
let plugin = Plugin.of_manifest ~functions ctx manifest |> Error.unwrap in

Need help?

If you've encountered a bug or think something is missing, please open an issue on the Extism GitHub repository.

There is an active community on Discord where the project maintainers and users can help you. Come hang out!