Std.MultiKeyMap.MultiKeyMap

type

This declaration introduces a public type.

What it does

A map keyed by two things, which can also be asked about either one alone.

The composite key by itself is not why this exists: Map[(A, B), V] already

works, because the runtime's order handles tuples, and a caller wanting only

"the value under this pair" should write that and stop here.

What a pair-keyed Map cannot do is answer about part of the key. Every

permission for one user, whatever the resource; every reading from one

sensor, whatever the hour; every override for one environment, whatever the

setting. Against Map[(A, B), V] each of those reads every entry and throws

most of them away, and the map gets slower at exactly the rate the data

grows.

So this keeps two indexes beside the entries — one grouping the pairs by

their first part, one by their second — and a partial lookup reads the group

it wants instead of the whole map. That is the cost too: three structures

updated on every write, in exchange for the two questions. A caller who never

asks a partial question is paying for indexes they do not read, and should

use a pair-keyed Map instead. This is written on the module because it is

the whole decision of whether to reach for it.

Read the signature

• This declaration has no value signature because it introduces a type or trait.