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Haskell SDK reference

Read time: 3 minutes
Last edited: Jan 29, 2024
Version 4 of the Haskell SDK replaces users with contexts

A context is a generalized way of referring to the people, services, machines, or other resources that encounter feature flags in your product.

Code samples on this page are from the two most recent SDK versions where they differ. To learn more about upgrading, read Haskell SDK 3.x to 3.0 migration guide.


This topic documents how to get started with the Haskell SDK, and links to reference information on all of the supported features.

SDK quick links

LaunchDarkly's SDKs are open source. In addition to this reference guide, we provide source, API reference documentation, and a sample application:

SDK API documentationSDK API docs
GitHub repositoryhaskell-server-sdk
Sample applicationHaskell
Published moduleHackage

Getting started

Follow the steps below to get started using the LaunchDarkly SDK in your Haskell application.

The root module of the SDK LaunchDarkly.Server re-exports the entire project.

Here is an example:

import LaunchDarkly.Server
The Haskell SDK uses an SDK key

The Haskell SDK uses an SDK key. Your environment's SDK key is available in the Projects tab of your Account settings page. To learn more about key types, read Keys.

After you import and install the SDK, create a single shared instance of Client. Specify your SDK key to authorize your application to connect to a particular environment within LaunchDarkly.

Here's how:

client :: IO Client
client = makeClient $ makeConfig "sdk-key-123abc"
Client must be a singleton

It's important to make Client a singleton for each LaunchDarkly project. The client instance maintains an internal state that allows LaunchDarkly to serve feature flags without making any remote requests. Do not instantiate a new client with every request.

If you have multiple LaunchDarkly projects, you can create one LDClient for each. In this situation, the clients operate independently. For example, they do not share a single connection to LaunchDarkly.

You can use client to check which variation a particular context will receive for a given feature flag. To learn more, read Evaluating flags and Flag evaluation reasons. For more information about how contexts are specified, read User and context configuration.

Here's how:

boolVariation client "flag-key-123abc" (makeContext "context-key-123abc" "context-kind") False

Shut down the client

Shut down the client when your application terminates. To learn more, read Shutting down.

Supported features

This SDK supports the following features: