Haskell SDK reference

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Last edited: Jun 14, 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.

Overview

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:

Resource	Location
SDK API documentation	SDK API docs
GitHub repository	haskell-server-sdk
Sample application	Haskell
Published module	Hackage

Get started

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

Install the SDK

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. Keys are specific to each project and environment. They are available from the Environments list for each project. To learn more about key types, read Keys.

Initialize the client

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"

To learn more about the specific configuration properties that are available in this SDK, read Config.

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.

Evaluate a context

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 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: