Exploring Life’s Ultimate Question with CEL

The ability to express and evaluate constraints, policies and conditions using a machine-readable language is not something new, the roots lie back in the Advanced Boolean Expression Language which was used in hardware verification.

Below is a sample program for describing the half-adder

module my_first_circuit; 
title 'ee200 assignment 1' 
EE200XY device 'XC4003E';
" input pins 
A, B pin 3, 5;
" output pins 
SUM, Carry_out pin 15, 18 istype 'com';
equations
SUM = (A & !B) # (!A & B) ; 
Carry_out = A & B;
end my_first_circuit;

evolving to software in the Java ecosystem as an Expression Language

<c:if test="${sessionScope.cart.numberOfItems > 0}">
  ...
</c:if>

notice how "${sessionScope.cart.numberOfItems > 0}" expression is represented as a string that allows the user to evaluate a condition depending on the value of numberOfItems in cart object from the session.

Common Expression Language is one such attempt from Google to write a language-agnostic expression language and evaluation engine.

A sample rule from cel-spec

account.balance >= transaction.withdrawal
    || (account.overdraftProtection
    && account.overdraftLimit >= transaction.withdrawal  - account.balance)