version 1 .

; Data types

WidgetTree =
/ <node @type NodeType @attributes WidgetAttributes @children [WidgetTree ...]>
/ <leaf @type LeafType @attributes WidgetAttributes>
.

NodeType = =column / =row .
WidgetAttributes = { symbol: any ...:... } .
LeafType =
/ =blank
/ =text
/ =slider
.

WidgetSelector =
/ @attributeEquals <= @key symbol @value any>
/ <and @selectors [WidgetSelector ...]>
.

Fill = @fixed double / <fill @weight int @rank int> .
Sizing = <sizing @ideal double @stretch Fill @shrink Fill> .
BoxSize = <box-size @horizontal Sizing @vertical Sizing> .

; Commonly set under key `id` in WidgetAttributes.
WidgetId = any .

Color = <rgba @red double @green double @blue double @alpha double> .

WidgetClientProtocol = WidgetInstance .

; Assertion
ShowWidget = <show
              @positions WidgetSelector
              @widget WidgetTree
              @order Ordering
              @client #!WidgetClientProtocol > .
Ordering = @double double / @string string .

; Assertion
WidgetInstance = <widget-instance @server #!any> .

; Assertion
Touch = <touch @widget WidgetId @touchId any> .

; Message
Click = <click @widget WidgetId> .

; Assertion
State = <state @widget WidgetId @key any @value any> .