Cmd.map Fractal Dashboard

Demonstrates Fractal Architecture using Cmd.map for component composition.

Problem

Without composition, a complex app needs one giant case statement handling every possible message from every child—the “God Reducer” anti-pattern. This doesn’t scale.

Solution

Cmd.map wraps child commands so their results route through parents:

# Child produces [:system_info, {stdout:, ...}]
child_cmd = SystemInfoWidget.fetch_cmd

# Parent wraps to produce [:stats, :system_info, {...}]
parent_cmd = Cmd.map(child_cmd) { |m| [:stats, *m] }

Each layer handles only its own messages. Parents pattern-match on the first element to route to the correct child.

Architecture

Dashboard (root)
├── StatsPanel
│   ├── SystemInfoWidget → Cmd.exec("uname -a", :system_info)
│   └── DiskUsageWidget  → Cmd.exec("df -h", :disk_usage)
└── NetworkPanel
    ├── PingWidget       → Cmd.exec("ping -c 1 localhost", :ping)
    └── UptimeWidget     → Cmd.exec("uptime", :uptime)

Hotkeys

Key	Action
`s`	Fetch system info
`d`	Fetch disk usage
`p`	Ping localhost
`u`	Fetch uptime
`q`	Quit

Key Concepts

Widget isolation: Each widget has its own Model, UPDATE, and fetch_cmd. It knows nothing about parents.
Message routing: Parents prefix child messages (:stats, :network) and pattern-match to route.
Recursive dispatch: Cmd.map delegates inner command execution to the runtime, then transforms the result.

Usage

ruby examples/widget_cmd_map/app.rb