mbs-script

mbs-script is a compact expression language implemented in C++20 and evaluated against an nlohmann::json context. It powers MantisBase's declarative rules — collection access control, filters, and computed logic — without pulling in a full JavaScript runtime for simple expressions.

• @variable lookups from a JSON context object
• Arithmetic, comparisons, booleans, string concat, and Python-style "x" * n repeat
• ^ exponentiation, // line comments, array literals, and method calls (.has(), .len(), etc.)
• Three entry points: test() for parse/semantic validation, eval_val() for full JSON results, eval_true() for boolean truthiness

MantisBase needed a safe, embeddable way to express dynamic rules inline — something lighter than shipping a JS engine for every @request.auth.id check. A purpose-built evaluator keeps rule evaluation fast, predictable, and easy to validate before runtime.

• Core: C++20 library (CMake), returns nlohmann::json values with null for invalid operand combinations
• Context: @name resolves against a caller-supplied JSON object (request data, record fields, auth claims)
• API: mbs::test(source), mbs::eval_val(source, context), mbs::eval_true(source, context)
• Integration: embedded directly in MantisBase as its expression evaluator; standalone repo with Catch2 tests and an example binary

Designing a tiny language for config-time rules means choosing strict null semantics over silent coercion — invalid mixes like "a" + 1 return JSON null rather than guessing. Separating test() from eval() lets MantisBase validate rule syntax at save time instead of failing at request time.