Hot and Cold Inlets

When an object has multiple inlets, not all of them behave the same way. The first inlet is hot — a message arriving there triggers the object to evaluate and send output immediately. All other inlets are cold — they quietly store the incoming value but produce no output on their own.

This design comes from Max and Pure Data. It gives you precise control over when a computation fires.

How It Works

Think of cold inlets as staging areas: you load them up with values, then fire the hot inlet when everything is ready.

Example: expr $1 + $2

[number 3] ──► inlet 1 (cold, $2)  ──┐
                                      ├──► [expr $1 + $2] ──► outlet
[number 5] ──► inlet 0 (hot,  $1)  ──┘

Step by step:

1. 3 arrives at inlet 1 (cold) — stored as $2, no output yet
2. 5 arrives at inlet 0 (hot) — triggers evaluation: 5 + 3 = 8 → sent to outlet

If both values arrived at inlet 0, you'd get an intermediate result (5 + 0 = 5) before the second value even arrives — almost always the wrong behavior.

Controlling Execution Order

When a single source feeds both inlets, you need to guarantee the cold inlet receives its value before the hot inlet fires. Use the trigger object for this.

trigger (or t) outputs its values right-to-left, so the rightmost outlet fires first:

[slider]
   │
   ▼
[t b a]
   │         └──► outlet 1 (value, fires first) ──► expr inlet 1 (cold)
   └──────────────► outlet 0 (bang,  fires second) ──► expr inlet 0 (hot)

1. The value reaches the cold inlet first
2. The bang triggers the hot inlet — by then the cold inlet is already loaded

Tip: Whenever you see unexpected or stale output from a multi-inlet object, check whether the cold inlets are being set before the hot inlet fires. A trigger object usually fixes it.

Objects That Use Hot/Cold Inlets

• expr — expression evaluator ($1, $2, …)
• map — JavaScript transformer
• filter — conditional message passing

See Also

• trigger — control message order
• Message Passing — how messages flow between objects