Type parameters let reusable functions preserve the relationship between input and output.
<div class="visual generic" data-step="0"><div class="visual-head"><strong>GENERIC TEMPLATE</strong><i></i></div><div class="visual-scene"><div class="generic-input mono">7</div><div class="generic-arrow">→</div><div class="generic-mold mono">identity<T></div><div class="generic-arrow">→</div><div class="generic-output mono">7 : number</div></div><div class="visual-caption"></div><div class="visual-dots"><b></b><b></b><b></b><b></b></div></div>.visual{width:min(95vw,760px);height:min(91vh,330px);padding:clamp(9px,2.5vmin,18px);display:flex;flex-direction:column;gap:clamp(6px,1.7vmin,11px);border:1px solid var(--line);border-radius:13px;background:var(--surface);font:500 clamp(12px,3.4vmin,16px)/1.25 var(--font-sans,-apple-system),sans-serif}
.visual-head{display:flex;align-items:center;justify-content:space-between;gap:8px;min-height:19px;color:var(--accent)}.visual-head strong{font-size:1em}.visual-head i{display:block;width:8px;height:8px;flex:none;border-radius:50%;background:var(--accent);animation:visual-pulse 1.3s infinite}@keyframes visual-pulse{50%{opacity:.25}}
.visual-scene{position:relative;flex:1;min-height:0;overflow:hidden}.visual-caption{min-height:2.4em;display:grid;place-items:center;text-align:center}.visual-dots{display:flex;justify-content:center;gap:4px}.visual-dots b{display:block;width:15px;height:3px;border-radius:3px;background:var(--line)}.visual-dots b.on{background:var(--accent)}
.visual-scene *{box-sizing:border-box}.visual-scene .mono{font-family:ui-monospace,monospace}.visual-scene .muted{color:var(--muted)}.visual-scene .lit{border-color:var(--accent)!important;background:color-mix(in srgb,var(--accent) 14%,var(--surface))!important;color:var(--fg)!important}
.generic .visual-scene{display:flex;align-items:center;justify-content:center;gap:4px}.generic-input,.generic-output{border:1px solid var(--line);border-radius:6px;background:var(--bg);padding:10px 5px;min-width:35px;text-align:center;font-size:12px;transition:transform .3s}.generic-mold{border:2px solid var(--accent);border-radius:9px;background:color-mix(in srgb,var(--accent) 12%,var(--surface));padding:13px 6px;text-align:center;font-size:12px}.generic-arrow{color:var(--accent);font-size:18px}.generic[data-step="1"] .generic-input{transform:translateX(6px)}.generic[data-step="2"] .generic-output,.generic[data-step="3"] .generic-output{transform:translateX(6px);border-color:var(--accent)}const root=document.querySelector('.visual'),cap=root.querySelector('.visual-caption'),dots=[...root.querySelectorAll('.visual-dots b')],captions=["Number enters one function","T becomes number","Output keeps number","String uses the same function"];let step=0;function advance(){root.dataset.step=String(step);cap.textContent=captions[step];dots.forEach((d,i)=>d.classList.toggle('on',i===step));if(root.classList.contains('generic')){root.querySelector('.generic-input').textContent=step===3?'"hi"':'7';root.querySelector('.generic-output').textContent=step===3?'"hi" : string':'7 : number'}if(root.classList.contains('union'))root.querySelector('.set-value').textContent=step%2?'value: "seven"':'value: 7';step=(step+1)%captions.length}advance();let timer=setInterval(advance,1100);root.addEventListener('pointerdown',()=>{clearInterval(timer);advance()})Generics leave a type parameter in a function instead of fixing one concrete type. The input selects a type, and that same relationship carries to the output.
The demo alternates number and string through one identity template. Add a type parameter when it connects positions, rather than as decoration.
Use it when a collection or transform function must preserve a relationship between input and result types.