Value Groups

A value group is a collection of values of the same type. Any number of constructors across an Fx application can feed values into a value group. Similarly, any number of consumers can read from a value group without knowing about the full list of producers.

flowchart TD
    group{{"[]Route"}}
    NewA & NewB & dots[...] & NewZ --> group
    group --> server[NewServeMux]
    group --> NewSiteMap

    style dots fill:none,stroke:none

Tip

Fx produces the values fed into a value group in a random order. Do not make any assumptions about value group ordering.

Using value groups

To learn how to use value groups, see,

• Feeding value groups
• Consuming value groups

Dependency strictness

Dependencies formed by value groups can be:

• strict: these are always consumed
• soft: these are consumed only if the corresponding constructor was requested elsewhere

By default, value group dependencies are strict.

Strict value groups

Strict value group dependencies are consumed by the value group regardless of whether their producers are otherwise used by the application.

Suppose a constructor NewFoo produces two values: A and B. Value A feeds into the value group []A, which is then consumed by function Run, and the application invokes function Run with fx.Invoke.

With strict value groups, Fx will run NewFoo to populate the []A group regardless of whether the application consumes the other result (B) directly or indirectly.

flowchart LR
    subgraph NewFoo
        A; B
    end
    subgraph "fx.Invoke"
        Run
    end
    A --> group{{"[]A"}} --> Run

Soft value groups

Soft value group dependencies are consumed by the value group only if the constructors that produce them were called by Fx anyway -- because the application consumes their other results directly or indirectly.

Suppose we have a setup similar to the previous section, except that the value group is soft.

flowchart LR
    subgraph NewFoo
        A; B
    end
    subgraph "fx.Invoke"
        Run
    end
    A -.-> group{{"[]A"}} --> Run

With soft value groups, Fx will run NewFoo to populate the []A group only if A or B are consumed by another component in the application directly or indirectly.

flowchart LR
    subgraph NewFoo
        A; B
    end
    subgraph "fx.Invoke"
        Run; Start
    end
    A -.-> group{{"[]A"}} --> Run
    B --> C --> Start