Lion expression model: JSON as code

Lion’s expression model is the foundation of the language: every program is a valid JSON value. There is no separate syntax to learn, no parser to invoke, and no serialization step. Numbers, booleans, null, strings, arrays, and objects are all Lion expressions, each evaluated according to its own rules. This homoiconic design means programs can be stored in databases, sent over HTTP, and generated by other systems — because they are already data.

Primitives
Arrays
Objects

Numbers, booleans, and null are self-evaluating — they return themselves unchanged. Strings are treated as environment references: if the string matches a key in the current environment the bound value is returned; if no binding exists the string returns as-is.

Evaluates to 42.

true

Evaluates to true.

null

Evaluates to null.

"x"

With { "x": 10 } in the environment, evaluates to 10. With no binding, evaluates to "x".

"number/add"

Resolves to the stdlib addition function because stdlib contains a "number/add" key.

Arrays are always executable expressions. The evaluator first checks whether the head element is a known special-form keyword. If it is, the matching special-form handler runs. Otherwise the head is evaluated to a function and called with the evaluated tail as arguments.Special form (define):

["define", "x", 42]

Stores 42 under "x" in the global environment and returns 42.Function call:

["number/add", 1, 2]

Resolves "number/add" from the environment, evaluates 1 and 2, then calls the function — returns 3.Nested calls:

["number/add", ["number/multiply", 2, 3], 4]

Inner call returns 6, outer call returns 10.Empty array:

[]

Evaluates to [] — an empty array is the one executable expression that passes through unchanged.

Objects preserve their keys and evaluate each value recursively. The result is a new object with the same key set but with every value replaced by its evaluated form. Keys are never evaluated.

{
  "sum": ["number/add", 1, 2],
  "ok": true
}

Evaluates to:

{
  "sum": 3,
  "ok": true
}

Deeply nested:

{
  "stats": {
    "total": ["number/add", 10, 20],
    "doubled": ["number/multiply", 2, 15]
  }
}

Evaluates to:

{
  "stats": {
    "total": 30,
    "doubled": 30
  }
}

Object keys must be non-empty strings. The key "__proto__" is disallowed by the schema to prevent prototype-pollution attacks on the host JavaScript environment.

Expression schema

Internally, Lion validates every value against the LionExpressionSchema before evaluation begins. The schema is a union of three branches:

export const LionExpressionSchema = Schema.Union(
  JsonPrimitiveSchema,        // number | boolean | null | string
  LionArrayExpressionSchema,  // readonly LionExpression[]
  LionRecordExpressionSchema  // { [key: string]: LionExpression }
);

Anything that does not conform — such as a JavaScript Date, undefined, or a class instance — is rejected at the boundary before evaluation starts.

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Core Concepts

Standard Library

Guides

Lion expression model: JSON as code

Expression schema

Get Started

Core Concepts

Standard Library

Guides

​Expression schema

Expression schema