Haskell

How to install and use the Extism Haskell PDK

Before you start

• This library is intended to be used with wasm32-wasi-ghc
• The Haskell PDK is different from the other PDKs because it requires WASI and the resulting plugins expose a single _start function instead of named functions. It is possible to export named Haskell functions, however because Haskell has a runtime that needs to be initialized it's not possible to call them directly.
• If you're geting linker errors about undefined Extism functions when compiling a plugin then the following arguments need to be passed to GHC: -optl -Wl,--allow-undefined (see extism-pdk.cabal)

Installation

Add to your cabal.project file:

source-repository-package
  type: git
  location: https://github.com/extism/extism.git
  subdir: haskell

source-repository-package
  type: git
  location: https://github.com/extism/haskell-pdk.git

Compiling to WebAssembly

Install the wasm-ready version of the Haskell toolchain, following these instructions, and run:

wasm32-wasi-cabal build

Example Usage

NOTE: All Haskell-based plug-ins will require an Extism host to enable WASI from its SDK.

Using JSON

CountVowels.hs

module Main where

import Extism.PDK
import Extism.PDK.JSON

isVowel c = 
  c == 'a' || c == 'A' ||
  c == 'e' || c == 'E' ||
  c == 'i' || c == 'I' ||
  c == 'o' || c == 'O' ||
  c == 'u' || c == 'U'

main = do
  -- Get input string from Extism host
  s <- inputString
  -- Calculate the number of vowels
  let count = length (filter isVowel s)
  -- Return a JSON object {"count": count} back to the host
  outputJSON $ object ["count" .= count]

Using Extism built-in HTTP

HTTPGet.hs

module Main where

import Extism.PDK
import Extism.PDK.HTTP

main = do
  -- Get URL from the host
  url <- inputString
  -- Create a new 'Request'
  let req = newRequest url
  -- Send the request, get a 'Response'
  res <- sendRequest req Nothing
  -- Save response body to memory
  outputMemory (memory res)

Using I/O

Hello.hs

module Main where

import Extism.PDK
import Data.Maybe

greet g n =
  outputString $ g ++ ", " ++ n

main = do
  -- Get a name from the Extism runtime
  name <- inputString
  -- Get  configured greeting
  greeting <- getConfig "greeting"
  -- Greet the user, if no greeting is configured then "Hello" is used
  greet (fromMaybe "Hello" greeting) name