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The CUE Data Constraint Language

Configure, Unify, Execute

CUE is an open source data constraint language which aims to simplify tasks involving defining and using data.

It is a superset of JSON, allowing users familiar with JSON to get started quickly.

What is it for?

You can use CUE to

• define a detailed validation schema for your data (manually or automatically from data)
• reduce boilerplate in your data (manually or automatically from schema)
• extract a schema from code
• generate type definitions and validation code
• merge JSON in a principled way
• define and run declarative scripts

How?

CUE merges the notion of schema and data. The same CUE definition can simultaneously be used for validating data and act as a template to reduce boilerplate. Schema definition is enriched with fine-grained value definitions and default values. At the same time, data can be simplified by removing values implied by such detailed definitions. The merging of these two concepts enables many tasks to be handled in a principled way.

Constraints provide a simple and well-defined, yet powerful, alternative to inheritance, a common source of complexity with configuration languages.

CUE Scripting

The CUE scripting layer defines declarative scripting, expressed in CUE, on top of data. This solves three problems: working around the closedness of CUE definitions (we say CUE is hermetic), providing an easy way to share common scripts and workflows for using data, and giving CUE the knowledge of how data is used to optimize validation.

There are many tools that interpret data or use a specialized language for a specific domain (Kustomize, Ksonnet). This solves dealing with data on one level, but the problem it solves may repeat itself at a higher level when integrating other systems in a workflow. CUE scripting is generic and allows users to define any workflow.

Tooling

CUE is designed for automation. Some aspects of this are:

• convert existing YAML and JSON
• automatically simplify configurations
• rich APIs designed for automated tooling
• formatter
• arbitrary-precision arithmetic
• generate CUE templates from source code
• generate source code from CUE definitions (TODO)

Download and Install

Release builds

Download the latest release from GitHub.

Run with Docker

docker run cuelang/cue version

Install using Homebrew

brew install cue-lang/tap/cue

Install from Source

You need Go 1.21 or later to install CUE from source:

go install cuelang.org/go/cmd/cue@latest

You can also clone the repository and build it directly via go install ./cmd/cue. Note that local builds lack version information, so you should inject the version string when building a release, such as:

git switch -d v0.9.0
go install -ldflags='-X cuelang.org/go/cmd/cue/cmd.version=v0.9.0' ./cmd/cue

Learning CUE

The fastest way to learn the basics is to follow the tour on the website.

More documentation including various tutorials can be found on the website.

References

• Language Specification: the official CUE Language specification
• Go API: the Go API on pkg.go.dev
• Builtin packages: builtin functions available from CUE programs
• cue CLI: the cue command line interface

Go release support policy

As a general rule, we support the two most recent major releases of Go, matching Go's security policy. For example, if CUE v0.7.0 is released when Go's latest version is 1.21.5, v0.7.x including any following bugfix releases will require Go 1.20 or later.

Contributing

To contribute, please read the Contribution Guide.

Code of Conduct

Guidelines for participating in CUE community spaces and a reporting process for handling issues can be found in the Code of Conduct.

Contact

• Ask questions via GitHub Discussions
• Chat with us on Slack and Discord
• Subscribe to our Community Calendar for community updates, demos, office hours, etc

Unless otherwise noted, the CUE source files are distributed under the Apache 2.0 license found in the LICENSE file.