hal provides an AWS Lambda Custom Runtime environment for your Haskell applications.
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Latest commit 1a93f90 Jan 12, 2019



A runtime environment for Haskell applications running on AWS Lambda.


This library uniquely supports different types of AWS Lambda Handlers for your needs/comfort with advanced Haskell. Instead of exposing a single function that constructs a Lambda, this library exposes many.

For lambdas that are pure and safe, then pureRuntime is ideal. It accepts a handler with the signature (FromJSON a, ToJSON b) => a -> b. This runtime guarantees that side-effects cannot occur.

For advanced use cases mRuntimeWithContext unlocks the full power of Monad Transformers. It accepts handlers with the signature (HasLambdaContext r, MonadCatch m, MonadReader r m, MonadIO m, FromJSON event, ToJSON result) => (event -> m result) This enables users to add caching logic or expose complex environments.

With numerous options in between these two, developers can choose the right balance of flexibility vs simplicity.


Measuring lambda performance is tricky, so investigation and optimization is ongoing. Current indications show a warm execution overhead of only ~20% more than the official Rust Runtime (a much lower level language).


While testing continues, we have executed over 30k test events without error caused by the runtime. Naive approaches lead to error rates well over 10%.

Table of Contents

Supported Platforms / GHC Versions

We currently support this library under the same environment that AWS Lambda supports.

Our CI currently targets the latest three LTS Stackage Versions, the latest three minor versions of GHC under Cabal (e.g. 8.6.x, 8.4.x, and 8.2.x), and GHC-head / Stackage nightly builds.

If you haven't already, adding docker: { enable: true } to your stack.yaml file will ensure that you're building a binary that can run in AWS Lambda.

Quick Start

This quick start assumes you have the following tools installed:

Add hal to your stack.yaml's extra-deps and enable Docker integration so that your binary is automatically compiled in a compatible environment for AWS. Also add hal to your project's dependency list (either project-name.cabal or package.yaml)

  - '.'
  - hal-0.1.0
# ...
  enable: true
# ...

Then, define your types and handler:

{-# LANGUAGE NamedFieldPuns #-}
{-# LANGUAGE DeriveGeneric #-}

module Main where

import AWS.Lambda.Runtime (pureRuntime)
import Data.Aeson (FromJSON, ToJSON)
import GHC.Generics (Generic)

data Request = Request {
  input :: String
} deriving (Generic)

instance FromJSON Request

data Response = Response {
  output :: String
} deriving (Generic)

instance ToJSON Response

idHandler :: Request -> Response
idHandler Request { input } = Response { output = input }

main :: IO ()
main = pureRuntime idHandler

Don't forget to define your CloudFormation stack:

# file: template.yaml
AWSTemplateFormatVersion: '2010-09-09'
Transform: 'AWS::Serverless-2016-10-31'
Description: Test for the Haskell Runtime.
    Type: 'AWS::Serverless::Function'
      Handler: NOT_USED
      Runtime: provided
      CodeUri: .stack-work/docker/_home/.local/bin/
      Description: My Haskell runtime.
      MemorySize: 128
      Timeout: 3

Finally, build, upload and test your lambda!

# Build the binary, make sure your executable is named `bootstrap`
stack build --copy-bins

# Create your function package
aws cloudformation package \
  --template-file template.yaml
  --s3-bucket your-existing-bucket > \

# Deploy your function
aws cloudformation deploy \
  --stack-name "hello-world-haskell" \
  --region us-west-2 \
  --capabilities CAPABILITY_IAM \
  --template-file deployment_stack.yaml

# Take it for a spin!
aws lambda invoke \
  --function-name your-function-name \
  --region us-west-2
  --payload '{"input": "foo"}'



Local Testing



docker pull fpco/stack-build:lts-12.21 #first build only
stack build --copy-bins


echo '{ "accountId": "byebye" }' | sam local invoke --region us-east-1