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This project parses and evaluates Common Expression Language (CEL) programs against some inputs.

cel-dart #

pub package Unit Tests

This project parses and evaluates Common Expression Language (CEL) programs against some inputs. For example, based on the code request.auth.claims.group=='admin' and a request object as input, the library will evaluate whether the statement is true or false. CEL (see the spec) is a language used by many security projects such as Firestore and Firebase Storage. This project is a simplified port of https://github.com/google/cel-go.

Usage #

import 'package:cel/cel.dart';

void main() {
  final input = "request.auth.claims.group == 'admin'";
  final e = Environment.standard();
  final ast = e.compile(input);
  final p = e.makeProgram(ast);
  print(p.evaluate({
    'request': {
      'auth': {
        'claims': {'group': 'admin'}
      }
    }
  }));
}

Prints out true.

Differences with cel-go #

The main difference is that cel-go supports checking types at compilation time, whereas we throw runtime errors at evaluation time. Also we don't support the timestamps nor durations Protobufs, type conversions and the type keyword.

Features #

This table is based on https://github.com/google/cel-spec/blob/master/doc/langdef.md.

CEL Literal Description Supported
null Null Literal
true and false Bool Literal
"abc" String Literal
-13, 0xff Int Literal
12u Uint Literal
12.6 Double Literal
b"abc" Bytes Literals
user.id == "abc" Operators See table below
Structures Description Supported
[a, b] List
{'name': 'cel', 35 : true} Map
timestamp google.protobuf.Timestamp
duration google.protobuf.Duration

Operators #

This table comes from https://firebase.google.com/docs/rules/rules-language#operators_and_operator_precedence.

Operator Description Supported
a.f field access
a() call
a[i] Index
!a, -a Unary negation
a/b, a%b, a*b Multiplicative operators
a+b, a-b Additive operators
a>b, a>=b Relational operators
a in b Existence in list or map
a is type Type comparison, where type can be bool, int, float, number, string, list, map, timestamp, duration, path or latlng
a==b, a!=b Comparison operators
a && b Conditional AND
a || b Conditional OR
a ? true_value : false_value Ternary expression

Functions #

From https://github.com/google/cel-spec/blob/master/doc/langdef.md#functions.

Symbol Type Description
!_ (bool) -> bool logical not
-_ (int) -> int negation
(double) -> double negation
_!=_ (A, A) -> bool inequality
_%_ (int, int) -> int arithmetic
(uint, uint) -> uint arithmetic untested
_&&_ (bool, bool) -> bool logical and
(bool, ...) -> bool logical and (variadic)
_*_ (int, int) -> int arithmetic
(uint, uint) -> uint arithmetic
(double, double) -> double arithmetic
_+_ (int, int) -> int arithmetic
(uint, uint) -> uint arithmetic
(double, double) -> double arithmetic
(string, string) -> string String concatenation.
(bytes, bytes) -> bytes bytes concatenation
(list(A), list(A)) -> list(A) List concatenation.
(google.protobuf.Timestamp, google.protobuf.Duration) -> google.protobuf.Timestamp arithmetic
(google.protobuf.Duration, google.protobuf.Timestamp) -> google.protobuf.Timestamp arithmetic
(google.protobuf.Duration, google.protobuf.Duration) -> google.protobuf.Duration arithmetic
_-_ (int, int) -> int arithmetic
(uint, uint) -> uint arithmetic
(double, double) -> double arithmetic
(google.protobuf.Timestamp, google.protobuf.Timestamp) -> google.protobuf.Duration arithmetic
(google.protobuf.Timestamp, google.protobuf.Duration) -> google.protobuf.Timestamp arithmetic
(google.protobuf.Duration, google.protobuf.Duration) -> google.protobuf.Duration arithmetic
_/_ (int, int) -> int arithmetic
(uint, uint) -> uint arithmetic
(double, double) -> double arithmetic
_<=_ (bool, bool) -> bool ordering
(int, int) -> bool ordering
(uint, uint) -> bool ordering
(double, double) -> bool ordering
(string, string) -> bool ordering
(bytes, bytes) -> bool ordering
(google.protobuf.Timestamp, google.protobuf.Timestamp) -> bool ordering
(google.protobuf.Duration, google.protobuf.Duration) -> bool ordering
_<_ (bool, bool) -> bool ordering
(int, int) -> bool ordering
(uint, uint) -> bool ordering
(double, double) -> bool ordering
(string, string) -> bool ordering
(bytes, bytes) -> bool ordering
(google.protobuf.Timestamp, google.protobuf.Timestamp) -> bool ordering
(google.protobuf.Duration, google.protobuf.Duration) -> bool ordering
_==_ (A, A) -> bool equality
_>=_ (bool, bool) -> bool ordering
(int, int) -> bool ordering
(uint, uint) -> bool ordering
(double, double) -> bool ordering
(string, string) -> bool ordering
(bytes, bytes) -> bool ordering
(google.protobuf.Timestamp, google.protobuf.Timestamp) -> bool ordering
(google.protobuf.Duration, google.protobuf.Duration) -> bool ordering
_>_ (bool, bool) -> bool ordering
(int, int) -> bool ordering
(uint, uint) -> bool ordering
(double, double) -> bool ordering
(string, string) -> bool ordering
(bytes, bytes) -> bool ordering
(google.protobuf.Timestamp, google.protobuf.Timestamp) -> bool ordering
(google.protobuf.Duration, google.protobuf.Duration) -> bool ordering
_?_:_ (bool, A, A) -> A The conditional operator. See above for evaluation semantics. Will evaluate the test and only one of the remaining sub-expressions.
_[_] (list(A), int) -> A list indexing.
(map(A, B), A) -> B map indexing.
in (A, list(A)) -> bool list membership.
(A, map(A, B)) -> bool map key membership.
|| (bool, bool) -> bool logical or
(bool, ...) -> bool logical or (variadic)
bool type(bool) type denotation
bytes type(bytes) type denotation
(string) -> bytes type conversion
contains string.(string) -> bool Tests whether the string operand contains the substring.
double type(double) type denotation
(int) -> double type conversion
(uint) -> double type conversion
(string) -> double type conversion
duration (string) -> google.protobuf.Duration Type conversion. Duration strings should support the following suffixes: "h" (hour), "m" (minute), "s" (second), "ms" (millisecond), "us" (microsecond), and "ns" (nanosecond). Duration strings may be zero, negative, fractional, and/or compound. Examples: "0", "-1.5h", "1m6s"
dyn type(dyn) type denotation
(A) -> dyn type conversion
endsWith string.(string) -> bool Tests whether the string operand ends with the suffix argument.
getDate google.protobuf.Timestamp.() -> int get day of month from the date in UTC, one-based indexing
google.protobuf.Timestamp.(string) -> int get day of month from the date with timezone, one-based indexing
getDayOfMonth google.protobuf.Timestamp.() -> int get day of month from the date in UTC, zero-based indexing
google.protobuf.Timestamp.(string) -> int get day of month from the date with timezone, zero-based indexing
getDayOfWeek google.protobuf.Timestamp.() -> int get day of week from the date in UTC, zero-based, zero for Sunday
google.protobuf.Timestamp.(string) -> int get day of week from the date with timezone, zero-based, zero for Sunday
getDayOfYear google.protobuf.Timestamp.() -> int get day of year from the date in UTC, zero-based indexing
google.protobuf.Timestamp.(string) -> int get day of year from the date with timezone, zero-based indexing
getFullYear google.protobuf.Timestamp.() -> int get year from the date in UTC
google.protobuf.Timestamp.(string) -> int get year from the date with timezone
getHours google.protobuf.Timestamp.() -> int get hours from the date in UTC, 0-23
google.protobuf.Timestamp.(string) -> int get hours from the date with timezone, 0-23
google.protobuf.Duration.() -> int get hours from duration
getMilliseconds google.protobuf.Timestamp.() -> int get milliseconds from the date in UTC, 0-999
google.protobuf.Timestamp.(string) -> int get milliseconds from the date with timezone, 0-999
google.protobuf.Duration.() -> int milliseconds from duration, 0-999
getMinutes google.protobuf.Timestamp.() -> int get minutes from the date in UTC, 0-59
google.protobuf.Timestamp.(string) -> int get minutes from the date with timezone, 0-59
google.protobuf.Duration.() -> int get minutes from duration
getMonth google.protobuf.Timestamp.() -> int get month from the date in UTC, 0-11
google.protobuf.Timestamp.(string) -> int get month from the date with timezone, 0-11
getSeconds google.protobuf.Timestamp.() -> int get seconds from the date in UTC, 0-59
google.protobuf.Timestamp.(string) -> int get seconds from the date with timezone, 0-59
google.protobuf.Duration.() -> int get seconds from duration
int type(int) type denotation
(uint) -> int type conversion
(double) -> int Type conversion. Rounds toward zero, then errors if result is out of range.
(string) -> int type conversion
(enum E) -> int type conversion
(google.protobuf.Timestamp) -> int Convert timestamp to int64 in seconds since Unix epoch.
list type(list(dyn)) type denotation
map type(map(dyn, dyn)) type denotation
matches (string, string) -> bool Matches first argument against regular expression in second argument.
string.(string) -> bool Matches the self argument against regular expression in first argument.
null_type type(null) type denotation
size (string) -> int string length
(bytes) -> int bytes length
(list(A)) -> int list size.
(map(A, B)) -> int map size.
startsWith string.(string) -> bool Tests whether the string operand starts with the prefix argument.
string type(string) type denotation
(int) -> string type conversion
(uint) -> string type conversion
(double) -> string type conversion
(bytes) -> string type conversion
(timestamp) -> string type conversion, using the same format as timestamp string parsing
(duration) -> string type conversion, using the same format as duration string parsing
timestamp (string) -> google.protobuf.Timestamp Type conversion of strings to timestamps according to RFC3339. Example: "1972-01-01T10:00:20.021-05:00"
type type(dyn) type denotation
(A) -> type(dyn) returns type of value
uint type(uint) type denotation
(int) -> uint type conversion
(double) -> uint Type conversion. Rounds toward zero, then errors if result is out of range.
(string) -> uint type conversion
E (for fully-qualified enumeration E) (int) -> enum E type conversion when in int32 range, otherwise error
(string) -> enum E type conversion for unqualified symbolic name, otherwise error

Additional information #

If you are curious how it was made, or want to contribute, you may find this reading list useful:

Architecture #

Here's the mechanism from CEL code (a String) to evaluation:

  1. The user instantiates an [Environment]. In cel-go, they can pass some environment variables. We have skipped porting this so far.
  2. The user calls [Environment.compile] with CEL code (a String), and gets back an Abstract Syntax Tree (AST).
    1. Under the hood, [Environment.compile] relies on [Parser], which itself uses [CELParser], an ANTLR generated Parser for CEL.
    2. [CELParser] converts the CEL code into a CEL tree (a [StartContext]).
    3. Then Parser traverses the CEL tree into an [Expr], which is the actual AST.
    4. Finally Environment wraps the [Expr] into an [Ast].
  3. The user instantiates a [Program] by passing the Environment and the AST. Upon initialization, the Program calls [Planner.plan], which traverses the AST and converts it into an [Interpretable] for later use.
  4. Whenever the user wants to evaluate the Program, they call [Program.evaluate] with some inputs (eg a [Map]), and get a value as a result. It evaluates the Interpretable using the inputs into a return value.

The meat of the code is in [Parser.visit] and [Planner.plan].

Implementation details #

  • Difference between [Value.value] and [Value.convertToNative]: While both are the same in the case of primitive wrappers such as [IntValue], [DoubleValue]... they are different for [ListValue] and [MapValue]. For example for a [ListValue], [ListValue.value] is a List<Value>, while [Value.convertToNative] will return List<non-Value type>.
  • environmentOptions and standardDeclarations don't actually do anything yet. In the future, they may be used to check whether some function has indeed been declared in Interpretable.planCall when it calls resolveFunction. Doing so might help throw an Exception early if the function name is not an declared function.
  • In cel-go, Parser.visit returns any. In cel-dart, we return Expr, making it more type safe.
  • How does a in b get processed? in is listed in standardOverloads. It is used in StdLibrary to add them to the Dispatcher during initialization. During evaluation, the planner finds the Overload implementation by calling Dispatcher.findOverload. Eventually, the CallExpr('@in') calls the [Overload] implementation with the call to contains.
  • In cel-go defines the Expr architecture with Protobuf, while this project defines Expr as native Dart. This is mostly to save time by avoiding a lot of boilerplate code. We might integrate Protobuf later if the need arises.

Re-generating CELParser #

  1. Run ./lib/src/parser/gen/generate.sh.
  2. Using Visual Studio Code, in CELParser.dart replace the regex \(\(1 << _la\) & (\d+)\) by bitwiseAnd(pow(2, _la), $1).
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This project parses and evaluates Common Expression Language (CEL) programs against some inputs.

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License

MIT (license)

Dependencies

antlr4, collection, equatable

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