Module space 

Search spaces: values, distributions, transforms, schemas.

The layer is four types deep and no deeper:

Type	Answers
ParamValue	what one parameter is set to, with its type intact
Distribution	which values are legal, and how to map them to [0, 1]
Assignment	what a whole trial was set to (sorted by name)
SpaceSchema	which parameters a study declares (in declaration order)

Two invariants run through all of them — the guide states them under the four kinds of parameter:

• Parameters are identified by stable string names, values are typed. Nothing is keyed on declaration order (the footgun that global-counter designs hit), and a boolean never becomes the integer 1 (the collapse that Optuna’s f64 union suffers).
• There is exactly one transform layer. Distribution::to_unit and Distribution::from_unit are the only implementations of log scaling, step snapping and categorical indexing in the codebase; a sampler that wants numeric vectors goes through them explicitly.

Declared conditional parameters (enabled_if) do not exist; a conditional is written define-by-run, by suggesting a parameter only on the branch that uses it.

Modules

dsl

The string DSL: a compact text notation for a Distribution.

Structs

Assignment

The parameters of one trial: names to typed values.

CatChoices

The labels and typed scalar payloads of a categorical parameter, in a fixed order.

GrowthDecision

One growth decision the replay took, in the order it took them.

GrowthReplay

The growth rule replayed from the seed.

Open

One parameter’s declared growth policy: the seed it starts from, which side(s) may move, and the numbers that govern and bound that movement.

ParamSpec

One declared parameter: a name and its support.

SpacePolicy

A study’s growth policy: which parameters are open, and how.

SpaceSchema

An ordered set of declared parameters.

Enums

Band

The crowding band at one side of a distribution — see band.

CatChoiceValue

One categorical choice’s durable scalar payload.

Distribution

The support of one parameter.

GrowthReason

Why a growth decision was taken or refused – the growth brakes and the two outcomes of the failure veto.

LimitBound

One side of an Open::limit call – which end of the stored limit a range argument sets. Never named by a caller: spelled with ..=/.. on the standard range types.

ParamValue

One parameter’s value, typed.

Scale

Whether a numeric range is walked linearly or logarithmically.

Side

Which end of a numeric range a measurement is about.

Sides

Which side(s) of a distribution a growth policy permits to move.

Spread

How Open::new_with_around’s seed is derived from a center value – the DSL’s around, one function name covering both scales via which keyword it is given.

ValueKind

The kind of a ParamValue, without its payload.

Constants

MAX_GRID_POINTS

The largest grid Distribution::grid_values will materialize.

Traits

Space

A typed configuration that doubles as a search space.

Functions

band

The crowding band at side of dist.

pressure

The crowding fraction on side: the share of the top set that band counts as “at the bound” there — pressure(side S) = |{ t in top set : t is in band(dist, S) }| / |top set|.

top_set

The completed trials that vote on where a study’s optimum is — the growth rule’s candidate and top-set construction.

widen

The ask-path interposition: the ask-time space with every policy-open parameter’s live (replayed) range in place of its seed.

widen_with

widen over an already-advanced replay — the cache path: the study handle keeps one GrowthReplay beside its view and advances it per decision point, so the rule replays over new trials only.