Hot and Cold Inlets

Objects with multiple inlets do not treat each inlet the same way. The first inlet is hot. A message at this inlet evaluates the object and sends output. The other inlets are cold. They store the incoming value but do not send output.

This design comes from Max and Pure Data. It lets you control when an object evaluates.

How It Works

Treat cold inlets as staging areas. Set their values before you send a message to the hot inlet.

Example: expr $1 + $2

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

To get this result:

1. Send 3 to inlet 1. The object stores it as $2 and does not send output.
2. Send 5 to inlet 0. The object evaluates 5 + 3 = 8 and sends the result to the outlet.

If both values arrive at inlet 0, the object sends an intermediate result. For example, it sends 5 + 0 = 5 before it receives the second value.

Controlling Execution Order

When one source feeds both inlets, set the cold inlet before you trigger the hot inlet. Use the trigger object to control this order.

trigger, or t, sends values from right to left. The rightmost outlet sends 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 after the cold inlet stores its value.

Tip: Check the message order when a multi-inlet object sends stale or unexpected output. Set cold inlets before you trigger the hot inlet. A trigger object can control the order.

Objects That Use Hot/Cold Inlets

• expr — Evaluate expressions with $1, $2, and more inputs.
• map — Transform messages with JavaScript.
• filter — Pass messages that meet a condition.

See Also

• trigger — Control message order.
• Message Passing — Learn how messages flow between objects.