#_ext.Select_

_ext.Select_ is the type behind the @select_ header and any payload field that wants the same "select fields from a result graph" behavior.

#Why It Is An Extension

The allowed shape is derived from the active function's Ok_ result payload and the nested structs and unions reachable from that result. That makes it context-sensitive in a way that a single static struct.* definition cannot express.

#Shape

_ext.Select_ is always an object. Its keys are selection targets:

• -> means "the active function result union".

• struct.SomeType means a reachable struct type.

• union.SomeType means a reachable union type.

Struct targets map to arrays of allowed field names:

{
  "struct.Profile": ["displayName", "avatarUrl"]
}

Union targets map tag names to arrays of allowed field names for that tag:

{
  "union.SearchResult": {
    "User": ["profile"],
    "Team": ["name"]
  }
}

The active result union can be selected through ->:

{
  "->": {
    "Ok_": ["profile", "summary"]
  },
  "struct.Profile": ["displayName"]
}

#How To Use It

• Send it in the @select_ header to trim fields from response payloads.

• You only need to specify the parts you want to narrow; omitted selections default to the full reachable shape.

• It applies recursively through arrays and objects when the nested value type is a selected struct or union.

• It does not let you omit function argument fields. Selection is for response graphs, not for changing request-link shapes.

#End-To-End Example

Suppose the schema contains:

- struct.ResultCard:
    title: string
    done!: boolean
- union.ResultItem:
    - Card:
        title: string
    - Note:
        body: string
- fn.selectNested: {}
  ->:
    - Ok_:
        card: struct.ResultCard
        item: union.ResultItem

If the server implementation would normally return:

[
  {},
  {
    "Ok_": {
      "card": {
        "title": "Ship docs",
        "done!": false
      },
      "item": {
        "Card": {
          "title": "Ship docs"
        }
      }
    }
  }
]

then this request:

[
  {
    "@select_": {
      "->": {
        "Ok_": ["card", "item"]
      },
      "struct.ResultCard": ["title"],
      "union.ResultItem": {
        "Card": []
      }
    }
  },
  {
    "fn.selectNested": {}
  }
]

changes the response shape to:

[
  {},
  {
    "Ok_": {
      "card": {
        "title": "Ship docs"
      },
      "item": {
        "Card": {}
      }
    }
  }
]

The data values did not change. Only the reachable fields selected by the header remained in the encoded response.

For a runnable minimal version of this pattern, see example/py-select.