CEL REPL

This is a simple tool for experimenting with CEL expressions and learning the syntax.

Usage

The REPL (Read Evaluate Print Loop) is implemented as a command line tool.

By default, the the input will be interpreted as a cel expression to evaluate.

Special commands (prefixed with '%') are used to update the evaluation environment.

An example session:

# from a cel-go clone
$ cd ./repl/main

$ go run .
CEL REPL
%exit or EOF to quit.

cel-repl> %let x = 10
cel-repl> %let y = {'abc': {'def': [1, 2, 3]}}
cel-repl> y.abc.def.filter(el, el < x)
[1 2 3] : list(int)
cel-repl> %delete x
cel-repl> y.abc.def.filter(el, el < x)
Expr failed:
ERROR: <input>:1:27: undeclared reference to 'x' (in container '')
 | y.abc.def.filter(el, el < x)
 | ..........................^
cel-repl> %declare x : int
cel-repl> y.abc.def.filter(el, el < x)
Expr failed:
no such attribute: id: 8, names: [x]
no such attribute: id: 8, names: [x] : list(int)
cel-repl> y.abc.def.filter(el, el < x || el > 0)
[1 2 3] : list(int)
cel-repl> %exit

Commands

compile

%compile compiles the input expression using the currently configured state into a protocol buffer text format representing the type-checked AST.

%compile <expr>

Example:

> %compile 3u
type_map:  {
    key:  1
    value:  {
        primitive:  UINT64
    }
}
source_info:  {
    location:  "<input>"
    line_offsets:  3
    positions:  {
        key:  1
        value:  0
    }
}
expr:  {
    id:  1
    const_expr:  {
        uint64_value:  3
    }
}

let

%let introduces or update a variable or function declaration and provide a definition (as another CEL expression). A type hint is optionally provided to check that the provided expression has the expected type.

%let <identifier> (: <type>)? = <expr>

Example:

%let y = 42

For functions, result types are mandatory:

%let <identifier> (<identifier> : <type>, ...) : <type> -> <expr>

Example:

%let oracle(x : int) : bool -> x == 42

Instance functions are declared as <type>.<identifier>(...): <type> and may reference this as the receiver instance.

Example:

> %let int.oracle() : bool -> this == 42
> 42.oracle()
true : bool
> 41.oracle()
false : bool

declare

%declare introduces or updates a variable or function declaration with no definition.

%declare <identifier> : <type>

%declare <identifier> (<identifier> : <type>, ...) : <type>

delete

%delete deletes a variable declaration

%delete <identifier>

eval

%eval evaluate an expression:

%eval <expr> or simply <expr>

status

%status prints a list of existing lets in the evaluation context.

load_descriptors

%load_descriptors loads a file descriptor set from file into the context. Message types from the file are available for use in later expressions as CEL structs.

Accepts an argument for the filedescriptor file format: --textproto or --binarypb.

example:

%load_descriptors --textproto "./testdata/attribute_context_fds.textproto"

option

%option sets an environment option. Options are specified with flags that may take string arguments.

--container <string> sets the expression container for name resolution.

--extension <extensionType> enables CEL extensions. Valid options are: strings, protos, math, encoders, optional, bindings, and all.

example:

%option --container 'google.protobuf'

%option --extension 'strings'

%option --extension 'all' (Loads all extensions)

reset

%reset drops all options and let expressions, returning the evaluator to a starting empty state.

Evaluation Model

The evaluator considers the let expressions and declarations in order, with functions defined before variables. Let expressions may refer to earlier expressions, but the reverse is not true. To prevent breaking dependant expressions, updates will fail if removing or changing a let prevents a later let expression from compiling. Let expressions are compiled when declared and evaluated before the expression in an %eval command.

Functions are implicitly defined before variables: let variables may refer to functions, but functions cannot refer to let variables.

Using curly-braces to indicate scopes, this looks like:

let sum (x : int, y : int) : int -> x + y
{
    let x = sum(2, 4)
    {
        let y = sum(x, 30)
        {
            eval sum(x, y) == 42
            // (x) + (x + 30)
            // (2 + 4) + ((2 + 4) + 30)
        }
    }
}

Installing

To build and install as a standalone binary:

$ git clone git@github.com:google/cel-go.git ./cel-go
$ cd ./cel-go/repl/main
$ go build -o repl .
# e.g. to your $PATH
$ mv ./repl <install location>