Skip to content


Repository files navigation

Extism Haskell Host SDK

This repo contains the Haskell package for integrating with the Extism runtime.

Note: If you're unsure what Extism is or what an SDK is see our homepage:


Documentation is available at


Install the Extism Runtime Dependency

For this library, you first need to install the Extism Runtime. You can download the shared object directly from a release or use the Extism CLI to install it.

Add the library to dune

Then add extism to your cabal file:

  build-depends: extism

Getting Started

This guide should walk you through some of the concepts in Extism and the Haskell bindings.

Creating A Plug-in

The primary concept in Extism is the plug-in. You can think of a plug-in as a code module stored in a .wasm file.

Since you may not have an Extism plug-in on hand to test, let's load a demo plug-in from the web:

module Main where
import Extism

main = do
  plugin <- unwrap <$> newPlugin (manifest [wasm]) [] True
  res <- unwrap <$> call plugin "count_vowels" "Hello, world!"
  putStrLn res
    wasm = wasmURL "GET" ""

-- Prints: {"count":3,"total":3,"vowels":"aeiouAEIOU"}"

Note: See the Manifest docs as it has a rich schema and a lot of options.

This plug-in was written in Rust and it does one thing, it counts vowels in a string. As such, it exposes one "export" function: count_vowels. We can call exports using

All exports have a simple interface of bytes-in and bytes-out. This plug-in happens to take a string and return a JSON encoded string with a report of results.

This library also allowes for conversion of input/outputs types using FromBytes and ToBytes

Plug-in State

Plug-ins may be stateful or stateless. Plug-ins can maintain state b/w calls by the use of variables. Our count vowels plug-in remembers the total number of vowels it's ever counted in the "total" key in the result. You can see this by making subsequent calls to the export:

ghci> unwrap <$> call plugin "count_vowels" "Hello, world!"
ghci> unwrap <$> call plugin "count_vowels" "Hello, world!"

These variables will persist until this plug-in is freed or you initialize a new one.


Plug-ins may optionally take a configuration object. This is a static way to configure the plug-in. Our count-vowels plugin takes an optional configuration to change out which characters are considered vowels. Example:

ghci> let manifest = manifest [wasm]
ghci> plugin <- unwrap <$> newPlugin manifest [] True
ghci> res <- (unwrap <$> call plugin "count_vowels" "Yellow, world!" :: String)
ghci> res

ghci> plugin <- withConfig (manifest [wasm]) [("vowels","aeiouyAEIOUY")] ;;
ghci> res <- (unwrap <$> call plugin "count_vowels" "Yellow, world!" :: String)
ghci> res

Host Functions

Let's extend our count-vowels example a little bit: we can intercept the results and adjust them before returning from the plugin using a hello_world host function with wasm/code-functions.wasm

Host functions allow us to grant new capabilities to our plug-ins from our application. They are simply some OCaml functions you write which can be passed down and invoked from any language inside the plug-in.

Using Extism.HostFunction.hostFunction we can define a host function that can be called from the guest plug-in.

In this example, we want to expose a single function to our plugin (in Haskell types): hello_world :: String -> String which will intercept the original result and replace it with a new one.

Let's load the manifest like usual but load up wasm/code-functions.wasm plug-in:

{-# LANGUAGE DeriveDataTypeable #-}

module Main where

import Extism
import Extism.HostFunction
import Extism.JSON
import Extism.Manifest (manifest, wasmFile)

newtype Count = Count {count :: Int} deriving (Data, Show)

hello currPlugin msg = do
  putStrLn . unwrap <$> input currPlugin 0
  putStrLn "Hello from Haskell!"
  putStrLn msg
  output currPlugin 0 (JSON $ Count 999)

main = do
  setLogFile "stdout" LogError
  f <- newFunction "hello_world" [ptr] [ptr] "Hello, again" hello
  plugin <- unwrap <$> newPlugin m [f] True
  id <- pluginID plugin
  print id
  JSON res <- (unwrap <$> call plugin "count_vowels" "this is a test" :: IO (JSON Count))
  print res
    m = manifest [wasmFile "wasm/code-functions.wasm"]
-- Prints: Count {count = 999}

Note: In order to write host functions you should get familiar with the methods on the Extism.HostFunction module.