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https://github.com/google/cel-go

Fast, portable, non-Turing complete expression evaluation with gradual typing (Go)
https://github.com/google/cel-go

cel expression expression-evaluator expression-language expression-parser go golang

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Fast, portable, non-Turing complete expression evaluation with gradual typing (Go)

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README 

        
# Common Expression Language



[![Go Report Card](https://goreportcard.com/badge/github.com/google/cel-go)](https://goreportcard.com/report/github.com/google/cel-go)

[![GoDoc](https://godoc.org/github.com/google/cel-go?status.svg)][6]



The Common Expression Language (CEL) is a non-Turing complete language designed

for simplicity, speed, safety, and portability. CEL's C-like [syntax][1] looks

nearly identical to equivalent expressions in C++, Go, Java, and TypeScript.



```java

// Check whether a resource name starts with a group name.

resource.name.startsWith("/groups/" + auth.claims.group)

```



```go

// Determine whether the request is in the permitted time window.

request.time - resource.age < duration("24h")

```



```typescript

// Check whether all resource names in a list match a given filter.

auth.claims.email_verified && resources.all(r, r.startsWith(auth.claims.email))

```



A CEL "program" is a single expression. The examples have been tagged as

`java`, `go`, and `typescript` within the markdown to showcase the commonality

of the syntax.



CEL is ideal for lightweight expression evaluation when a fully sandboxed

scripting language is too resource intensive. To get started, try the

[Codelab](https://codelabs.developers.google.com/codelabs/cel-go/index.html).



A dashboard that shows results of cel-go conformance tests can be found

[here](https://k8s-testgrid.appspot.com/google-cel#cel-go).



---



* [Overview](#overview)

  * [Environment Setup](#environment-setup)

  * [Parse and Check](#parse-and-check)

  * [Macros](#macros)

  * [Evaluate](#evaluate)

  * [Errors](#Errors)

* [Examples](examples/README.md)

* [Install](#install)

* [Common Questions](#common-questions)

* [License](#license)



---



## Overview



Determine the variables and functions you want to provide to CEL. Parse and

check an expression to make sure it's valid. Then evaluate the output AST

against some input. Checking is optional, but strongly encouraged.



### Environment Setup



Let's expose `name` and  `group` variables to CEL using the `cel.Declarations`

environment option:



```go

import "github.com/google/cel-go/cel"



env, err := cel.NewEnv(

    cel.Variable("name", cel.StringType),

    cel.Variable("group", cel.StringType),

)

```



That's it. The environment is ready to be used for parsing and type-checking.

CEL supports all the usual primitive types in addition to lists, maps, as well

as first-class support for JSON and Protocol Buffers.



### Parse and Check



The parsing phase indicates whether the expression is syntactically valid and

expands any macros present within the environment. Parsing and checking are

more computationally expensive than evaluation, and it is recommended that

expressions be parsed and checked ahead of time.



The parse and check phases are combined for convenience into the `Compile`

step:



```go

ast, issues := env.Compile(`name.startsWith("/groups/" + group)`)

if issues != nil && issues.Err() != nil {

    log.Fatalf("type-check error: %s", issues.Err())

}

prg, err := env.Program(ast)

if err != nil {

    log.Fatalf("program construction error: %s", err)

}

```



The `cel.Program` generated at the end of parse and check is stateless,

thread-safe, and cachable.



Type-checking in an optional, but strongly encouraged, step that can reject some

semantically invalid expressions using static analysis. Additionally, the check

produces metadata which can improve function invocation performance and object

field selection at evaluation-time.



#### Macros



Macros are optional but enabled by default. Macros were introduced to

support optional CEL features that might not be desired in all use cases

without the syntactic burden and complexity such features might desire if

they were part of the core CEL syntax. Macros are expanded at parse time and

their expansions are type-checked at check time.



For example, when macros are enabled it is possible to support bounded

iteration / fold operators. The macros `all`, `exists`, `exists_one`, `filter`,

and `map` are particularly useful for evaluating a single predicate against

list and map values.



```javascript

// Ensure all tweets are less than 140 chars

tweets.all(t, t.size() <= 140)

```



The `has` macro is useful for unifying field presence testing logic across

protobuf types and dynamic (JSON-like) types.



```javascript

// Test whether the field is a non-default value if proto-based, or defined

// in the JSON case.

has(message.field)

```



Both cases traditionally require special syntax at the language level, but

these features are exposed via macros in CEL.



### Evaluate



Now, evaluate for fun and profit. The evaluation is thread-safe and side-effect

free. Many different inputs can be sent to the same `cel.Program` and if fields

are present in the input, but not referenced in the expression, they are

ignored.



```go

// The `out` var contains the output of a successful evaluation.

// The `details' var would contain intermediate evaluation state if enabled as

// a cel.ProgramOption. This can be useful for visualizing how the `out` value

// was arrive at.

out, details, err := prg.Eval(map[string]interface{}{

    "name": "/groups/acme.co/documents/secret-stuff",

    "group": "acme.co"})

fmt.Println(out) // 'true'

```



#### Partial State



What if `name` hadn't been supplied? CEL is designed for this case. In

distributed apps it is not uncommon to have edge caches and central services.

If possible, evaluation should happen at the edge, but it isn't always possible

to know the full state required for all values and functions present in the

CEL expression.



To improve the odds of successful evaluation with partial state, CEL uses

commutative logical operators `&&`, `||`. If an error or unknown value (not the

same thing) is encountered on the left-hand side, the right hand side is

evaluated also to determine the outcome. While it is possible to implement

evaluation with partial state without this feature, this method was chosen

because it aligns with the semantics of SQL evaluation and because it's more

robust to evaluation against dynamic data types such as JSON inputs.



In the following truth-table, the symbols `` and `` represent error or

unknown values, with the `?` indicating that the branch is not taken due to

short-circuiting. When the result is `` this means that the both args

are possibly relevant to the result.



| Expression          | Result   |

|---------------------|----------|

| `false && ?`        | `false`  |

| `true && false`     | `false`  |

| ` && false`      | `false`  |

| `true && true`      | `true`   |

| `true && `       | ``    |

| ` && true`       | ``    |

| ` && `        | `` |

| `true \|\| ?`       | `true`   |

| `false \|\| true`   | `true`   |

| ` \|\| true`     | `true`   |

| `false \|\| false`  | `false`  |

| `false \|\| `    | ``    |

| ` \|\| false`    | ``    |

| ` \|\| `      | `` |



In the cases where unknowns are expected, `cel.EvalOptions(cel.OptTrackState)`

should be enabled. The `details` value returned by `Eval()` will contain the

intermediate evaluation values and can be provided to the `interpreter.Prune`

function to generate a residual expression. e.g.:



```cpp

// Residual when `name` omitted:

name.startsWith("/groups/acme.co")

```



This technique can be useful when there are variables that are expensive to

compute unless they are absolutely needed. This functionality will be the

focus of many future improvements, so keep an eye out for more goodness here!



### Errors



Parse and check errors have friendly error messages with pointers to where the

issues occur in source:



```sh

ERROR: :1:40: undefined field 'undefined'

    | TestAllTypes{single_int32: 1, undefined: 2}

    | .......................................^`,

```



Both the parsed and checked expressions contain source position information

about each node that appears in the output AST. This information can be used

to determine error locations at evaluation time as well.



## Install



CEL-Go supports `modules` and uses semantic versioning. For more info

see the [Go Modules](https://github.com/golang/go/wiki/Modules) docs.



And of course, there is always the option to build from source directly.



## Common Questions



### Why not JavaScript, Lua, or WASM?



JavaScript and Lua are rich languages that require sandboxing to execute

safely. Sandboxing is costly and factors into the "what will I let users

evaluate?" question heavily when the answer is anything more than O(n)

complexity.



CEL evaluates linearly with respect to the size of the expression and the input

being evaluated when macros are disabled. The only functions beyond the

built-ins that may be invoked are provided by the host environment. While

extension functions may be more complex, this is a choice by the application

embedding CEL.



But, why not WASM? WASM is an excellent choice for certain applications and

is far superior to embedded JavaScript and Lua, but it does not have support

for garbage collection and non-primitive object types require semi-expensive

calls across modules. In most cases CEL will be faster and just as portable

for its intended use case, though for node.js and web-based execution CEL

too may offer a WASM evaluator with direct to WASM compilation.



### Do I need to Parse _and_ Check?



Checking is an optional, but strongly suggested, step in CEL expression

validation. It is sufficient in some cases to simply Parse and rely on the

runtime bindings and error handling to do the right thing.



### Where can I learn more about the language?



* See the [CEL Spec][1] for the specification and conformance test suite.

* Ask for support on the [CEL Go Discuss][2] Google group.



### Where can I learn more about the internals?



* See [GoDoc][6] to learn how to integrate CEL into services written in Go.

* See the [CEL C++][3] toolchain (under development) for information about how

  to integrate CEL evaluation into other environments.



### How can I contribute?



* See [CONTRIBUTING.md](./CONTRIBUTING.md) to get started.

* Use [GitHub Issues][4] to request features or report bugs.



### Some tests don't work with `go test`?



A handful of tests rely on [Bazel][5]. In particular dynamic proto support

at check time and the conformance test driver require Bazel to coordinate

the test inputs:



```sh

bazel test ...

```



## License



Released under the [Apache License](LICENSE).



Disclaimer: This is not an official Google product.



[1]:  https://github.com/google/cel-spec

[2]:  https://groups.google.com/forum/#!forum/cel-go-discuss

[3]:  https://github.com/google/cel-cpp

[4]:  https://github.com/google/cel-go/issues

[5]:  https://bazel.build

[6]:  https://godoc.org/github.com/google/cel-go