Module expr 

Arithmetic / logical expression language for grammar arguments.

Every numeric argument position in the grammar (extrusion heights, split sizes, scale factors, roof parameters, …) accepts an expression instead of a bare literal. Expressions are evaluated at derivation time against the current shape’s EvalCtx — its scope extents, split position, depth, bound rule parameters, and grammar attributes — so one rule adapts to every scope it is applied to.

Extrude(rand(8, 14))
Split(Y) { FloorH: Ground | ~1: Upper }          // FloorH = const/attr
Scale(scope.x * 0.5, 1, 1)
when(split.i == 0): CornerBay | else: MidBay

Design notes:

• Floats only. Booleans are CGA-style floats: comparisons yield 1.0 / 0.0, and any non-zero value is truthy. This keeps one value type through parameters, attributes, and genetics.
• Deterministic randomness. rand(..) draws from the per-shape RNG stream supplied by the interpreter, so the same seed derives the same model. Short-circuit && / || skip RHS draws by design — divergence is data-driven and reproducible.
• Hard failure over silent nonsense. Division by zero, non-finite results, unknown identifiers, and bad function domains all return ShapeError rather than propagating NaNs into scope math.

Structs

EvalCtx

Per-shape evaluation context.

Enums

BinOp

Binary operators, lowest section = lowest precedence.

Expr

An argument expression, evaluated per shape at derivation time.

Func

Built-in functions.

UnaryOp

Unary operators.

Var

Built-in read-only variables.

Constants

MAX_EXPR_DEPTH

Maximum parenthesis/recursion depth inside one expression. Bounds nom’s recursion during parsing (a ((((…)))) bomb would otherwise overflow the stack long before node counting runs).

MAX_EXPR_NODES

Maximum number of AST nodes in a single expression (DoS hardening). Checked after parsing via Expr::node_count; grammar-level integration rejects any argument expression exceeding this.

Functions

parse_expr

nom-style entry point: parses one expression, leaving trailing input. Enforces MAX_EXPR_NODES before returning.

parse_expr_str

Whole-string entry point: the entire input must be one expression.