cel is a pure Ruby implementation of Google Common Expression Language, https://opensource.google/projects/cel.

The Common Expression Language (CEL) implements common semantics for expression evaluation, enabling different applications to more easily interoperate.

```ruby
require "cel"

# set the environment
env = Cel::Environment.new(name: :string, group: :string)

# parse
ast = env.compile('name.startsWith("/groups/" + group)')

# check
prg = env.program(ast)
# evaluate
prg.evaluate(name: Cel::String.new("/groups/acme.co/documents/secret-stuff"),
    group: Cel::String.new("acme.co")) #=> true

# or do it all in one go

env.evaluate('name.startsWith("/groups/" + group)',
  name: Cel::String.new("/groups/acme.co/documents/secret-stuff"),
  group: Cel::String.new("acme.co")
)

```

Here are the updates since the last release:

# [0.3.0] - 2025-06-12

### Features

* Integration with `google-protobuf` (optional dependency) by evaluating messages to protobuf stubs, and making them an integral part of the CEL type set. Some examples:
  * auto-conversion of protobuf wrapper types.
  * enums support

### Improvements

* More correct parsing, checking and evaluation of CEL expressions, thanks to the issues identified by conformance tests.

### Chore

* Incorporation of the oficial cel-spec conformance tests into the test suite, to ensure feature correctness.
* `bigdecimal` added as explicit dependency.

## [0.2.3] - 2023-09-19

### Bugfixes

* `cel` is loadable without the need to install `google/protobuf` again.
+ expressions containing te operator `<=` were failing while parsing.

## [0.2.2] - 2023-08-17

* Reimplements `Cel::Literal#==` in a simpler way, to avoid several comparison issues arising from the previous implementation.
* fix initialization of `Cel::Duration` and `Cel::Timestamp` from string notation.
* fix protobuf-to-cel parsing of negative literals.
* more consistent usage of Cel types internally.
* fix condition clause evaluation.


## [0.2.1] - 2023-07-26


### Improvements

Collection type declarations are now better supported, i.e. declaring "an array of strings" is now possible:

```ruby
Cel::Types[:list, :string] # array of strings
Cel::Environment.new(
    names: Cel::Types[:list, :string],
    owned_by: Cel::Types[:map, :string] # hash map of string keys
)
```

### Bugfixes

Collections passed as variables of an expression are now correctly cast to Cel types:

```ruby
env.evaluate("a", { a: [1, 2, 3] }) #=> now returns a Cel::List
```

Conversely, custom functions now expose ruby types in the blocks, instead of its Cel counterparts:

```ruby
func = Cel::Function(:list, :list, return_type: :list) do |a, b|
  # a is now a ruby array, b as well
  a & b
  # function returns a ruby array, will be converted to a Cel::List for use in the expression
end
```