Common Expression Language (Gleam)

The Common Expression Language (CEL) is a non-Turing complete language designed for simplicity, speed, safety, and portability. CEL's C-like syntax looks nearly identical to equivalent expressions in C++, Go, Java, and TypeScript. CEL is ideal for lightweight expression evaluation when a fully sandboxed scripting language is too resource intensive.

// Check whether a resource name starts with a group name.
resource.name.startsWith("/groups/" + auth.claims.group)

// Determine whether the request is in the permitted time window.
request.time - resource.age < duration("24h")

// Check whether all resource names in a list match a given filter.
auth.claims.email_verified && resources.all(r, r.startsWith(auth.claims.email))

Usage

gleam add cel@0.3

import gleam/io

import cel/interpreter

pub fn main() {
  let source =
    "[a, b, c].map(x, x + 2).filter(x, x > 4)[0] == 5 ? 'wibble' : 'wobble'"

  let ctx =
    interpreter.default_context()
    |> interpreter.insert_variable("a", interpreter.Int(1))
    |> interpreter.insert_variable("b", interpreter.Int(3))
    |> interpreter.insert_variable("c", interpreter.Int(5))

  let assert Ok(program) = interpreter.new(source)
  let assert Ok(interpreter.String(answer)) = interpreter.execute(program, ctx)

  io.println("execution result: " <> answer)
  // execution result: wibble
}

Further documentation can be found at https://hexdocs.pm/cel.

Development

gleam test  # Run the tests

State

This library is still very early in its development. The current state and planned future work:

• Lexing
• Parsing
• Expression evaluation
• Variable context resolution
• Functions/macros
• Context provision through Dynamic
• Type checking
• Field inits (ident{"a": 5})
• Duration + Timestamp
• Value → JSON (separate package)
• Value → MsgPack (separate package)
• (Maybe) Serializable AST