Parameter Model¶

class Parameter(BaseModel):
    value: float                # initial value
    min: float = -inf
    max: float = inf
    vary: bool = True           # False → fixed constant; ignored when expr is set
    expr: str | None = None     # constraint expression; evaluated every solver iteration
    scale: float | None = None  # solver step-size hint; None → 1.0 (identity, no-op)

name is the dict key in ModelNodeSpec.parameters — not duplicated as a field.

vary is ignored when expr is set

vary is ignored whenever expr is set — the engine always derives the value from the expression and excludes the parameter from the free set regardless of vary's value. There is no validator requiring vary=True when expr is set; vary simply has no effect in that case.

Three binding kinds resolved at compile time (free_mask = vary AND expr is None, spectrafit-graph::compiler):

Kind	vary	expr	Behaviour
Free	True	None	Element of the optimisation vector
Fixed	False	None	Constant; never updated
Expr	any	set	Derived from expression every iteration (Rust `TiedPlan`) — `vary` is ignored

Bounds are reflected, not clamped

Bounds (min, max) are enforced by reflective projection, not clamping: a step that overshoots a bound is mirrored back into range (p < lo → 2*lo - p, and symmetrically at hi), with an extreme overshoot parked at the violated bound instead of reflecting past the opposite one. This runs in LmProblem::apply_free_params (crates/spectrafit-solver/src/lm_problem.rs), called from both solver front-ends' set_params, not inside residuals(). scale is applied as an internal change-of-variables preconditioning: the solver works on theta' = theta / scale (LmProblem::scales, apply_free_params/scale_columns_rowmajor in crates/spectrafit-solver/src/lm_problem.rs), not forwarded to any external x_scale field — the levenberg-marquardt crate this workspace vendors has no such field.

Parameter Model¶

See also¶