Reducer pattern

<div class="scene"><div class="formula"><span id="action">dispatch add</span><span>→</span><span>reducer</span><span>→</span><b id="count">0 tasks</b></div><div class="tasklist"><div class="task" id="taskA">Task A <i></i></div><div class="task" id="taskB">Task B <i></i></div></div><div class="label" id="rule">state + action → next state</div></div>

.scene{width:min(94vw,800px);height:min(86vh,320px);position:relative;overflow:hidden;display:flex;flex-direction:column;justify-content:space-between;border:1px solid var(--line);border-radius:14px;background:var(--surface);padding:clamp(9px,2.4vmin,20px);font:500 clamp(15px,4vmin,18px)/1.3 var(--font-sans,-apple-system,BlinkMacSystemFont,"Segoe UI",sans-serif)}.scene b,.scene strong{font-weight:700}.label{color:var(--muted)}.code{font-family:ui-monospace,SFMono-Regular,Consolas,monospace}.pill{border:1px solid var(--line);border-radius:8px;padding:.4em .65em;background:var(--bg)}.active{border-color:var(--accent)!important;color:var(--accent)!important;background:color-mix(in srgb,var(--accent) 10%,var(--surface))!important}.formula{display:flex;justify-content:space-between;align-items:center;gap:3px;font:700 clamp(12px,3vmin,15px) ui-monospace,monospace}.formula b{color:var(--accent)}.tasklist{flex:1;min-height:55px;display:flex;gap:8px;align-items:center}.task{flex:1;min-height:42px;border:1px solid var(--line);border-radius:7px;display:flex;align-items:center;justify-content:space-between;padding:.3em .5em;opacity:.25;transition:opacity .3s}.task.visible{opacity:1}.task i{width:12px;height:12px;border:1px solid var(--accent);border-radius:3px}.task.done i{background:var(--accent)}.task.done{text-decoration:line-through;color:var(--muted)}.label{text-align:center;font-size:clamp(12px,3vmin,15px)}

const actions=["add A","add B","complete A","reset"];let step=0;function tick(){const action=actions[step];document.getElementById("action").textContent="dispatch "+action;document.getElementById("count").textContent=["1 task","2 tasks","2 tasks","0 tasks"][step];document.getElementById("taskA").classList.toggle("visible",step<3);document.getElementById("taskB").classList.toggle("visible",step===1||step===2);document.getElementById("taskA").classList.toggle("done",step===2);document.getElementById("rule").textContent=step===2?"complete returns a new state":"state + action → next state";step=(step+1)%4}tick();setInterval(tick,1000)