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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
Install using Homebrew

Using Homebrew, you can install using the CUE Homebrew tap:

brew install cuelang/tap/cue

Install From Source

If you already have Go installed, the short version is:

go get -u cuelang.org/go/cmd/cue

This will install the cue command line tool.

For more details see Installing CUE.

Learning CUE

The fastest way to learn the basics is to follow the tutorial on basic language constructs.

A more elaborate tutorial demonstrating of how to convert and restructure an existing set of Kubernetes configurations is available in written form.

References
Contributing

Our canonical Git repository is located at https://cue.googlesource.com.

To contribute, please read the Contribution Guide.

To report issues or make a feature request, use the issue tracker.

Changes can be contributed using Gerrit or Github pull requests.

Contact

You can get in touch with the cuelang community in the following ways:


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

This is not an officially supported Google product.

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Documentation

The Go Gopher

There is no documentation for this package.

Source Files

Directories

Path Synopsis
cmd/cue
cmd/cue/cmd
cmd/cue/cmd/interfaces
cue Package cue is a transition package for supporting the cue.Value API.
cue.mod
cue/ast Package ast declares the types used to represent syntax trees for CUE packages.
cue/ast/astutil
cue/build Package build defines data types and utilities for defining CUE configuration instances.
cue/encoding Package encoding provides support for managing data format files supported by CUE.
cue/errors Package errors defines shared types for handling CUE errors.
cue/format Package format implements standard formatting of CUE configurations.
cue/literal Package literal implements conversions to and from string representations of basic data types.
cue/load Package load loads CUE instances.
cue/parser Package parser implements a parser for CUE source files.
cue/scanner Package scanner implements a scanner for CUE source text.
cue/token Package token defines constants representing the lexical tokens of the Go programming language and basic operations on tokens (printing, predicates).
cuego Package cuego allows using CUE constraints in Go programs.
encoding Package encoding contains subpackages to convert CUE to and from byte-level and textual representations.
encoding/gocode Package gocode defines functions for extracting CUE definitions from Go code and generating Go code from CUE values.
encoding/gocode/gocodec Package codec converts Go to and from CUE and validates Go values based on CUE constraints.
encoding/json Package json converts JSON to and from CUE.
encoding/jsonschema Package jsonschema converts JSON Schema to CUE JSON Schema data is presented in CUE, so any of the supported encodings that can represent JSON Schema data can be used as a source.
encoding/openapi Package openapi provides functionality for mapping CUE to and from OpenAPI v3.0.0.
encoding/protobuf Package protobuf defines functionality for parsing protocol buffer definitions and instances.
encoding/yaml Package yaml converts YAML encodings to and from CUE.
internal Package internal exposes some cue internals to other packages.
internal/builtin
internal/cli
internal/cmd/qgo qgo builds CUE builtin packages from Go packages.
internal/copy Package copy provides utilities to copy files and directories.
internal/core/adt Package adt represents partially and fully evaluated CUE types.
internal/core/compile
internal/core/convert Package convert allows converting to and from Go values and Types.
internal/core/debug Package debug prints a given ADT node.
internal/core/eval Package eval contains the high level CUE evaluation strategy.
internal/core/export
internal/core/runtime
internal/core/subsume Package subsume defines various subsumption relations.
internal/core/validate Package validate collects errors from an evaluated Vertex.
internal/cuetest
internal/cuetxtar
internal/diff
internal/encoding
internal/encoding/json
internal/encoding/yaml
internal/filetypes
internal/source Package source contains utility functions that standardize reading source bytes across cue packages.
internal/str Package str provides string manipulation utilities.
internal/task Package task provides a registry for tasks to be used by commands.
internal/third_party/yaml Package yaml implements YAML support for the Go language.
internal/walk Package walk allows walking over CUE values.
pkg Package pkg define CUE standard packages.
pkg/crypto/md5
pkg/crypto/sha1
pkg/crypto/sha256
pkg/crypto/sha512
pkg/encoding/base64 Package base64 implements base64 encoding as specified by RFC 4648.
pkg/encoding/csv
pkg/encoding/hex
pkg/encoding/json
pkg/encoding/yaml
pkg/gen Package gen is a command that can be used to bootstrap a new builtin package directory.
pkg/html
pkg/internal
pkg/internal/builtintest
pkg/list Package list contains functions for manipulating and examining lists.
pkg/math
pkg/math/bits
pkg/net Package net provides net-related type definitions.
pkg/path
pkg/regexp
pkg/strconv
pkg/strings Package strings implements simple functions to manipulate UTF-8 encoded strings.package strings.
pkg/struct Package struct defines utilities for struct types.
pkg/text/tabwriter
pkg/text/template
pkg/time Package time defines time-related types.
pkg/tool Package tool defines statefull operation types for cue commands.
pkg/tool/cli Package cli provides tasks dealing with a console.
pkg/tool/exec Package exec defines tasks for running commands.
pkg/tool/file Package file provides file operations for cue tasks.
pkg/tool/http Package http provides tasks related to the HTTP protocol.
pkg/tool/os Package os defines tasks for retrieving os-related information.
tools/fix Package fix contains functionality for writing CUE files with legacy syntax to newer ones.
tools/trim Package trim removes definitions that may be inferred from templates.