JLCEL Course

CH05-L04 · applied

The map() macro

Transform every input-list item into a new value.

General CELRuns hereDocumented
What will I learn?

You can use map() for transformation and distinguish it from filter() selection.

Where will I try it?

Directly below, in the browser playground.

What do I do?

Run both modes, compare where their values come from, then complete the task.

Try CEL · browser pilot

Try both ways to supply values

Define local values inside one CEL expression, or provide variables as JSON input. Everything runs in your browser.

CEL pilot subset
One self-contained CEL expressioncel.bind() gives a local name to a value and returns the result of the final expression.

cel.bind()cel.bind() is an optional official extension. It is not part of the core CEL language and may not be enabled by every host application.

Tip: press Ctrl/⌘ + Enter to run.
Result

Press Run CEL to evaluate the active example.

Pilot scope: literals, explicit conversions, local cel.bind() variables, JSON variables, lists, maps, presence, collection macros, core string checks, time values, arithmetic, comparisons, logic, and the conditional operator.

What you will do

You will run the prepared expression in the playground above, change one input at a time, and explain the result. Start in CEL only, then repeat the same rule in CEL + JSON.

How it works

map() evaluates a transformation for every item and returns the transformed results in the same order. It always returns a list. Unlike filter(), it does not decide which items survive; it decides what each output item becomes.

Read the prepared example

The CEL + JSON rule is:

prices.map(price, price * 2.0)

Its input is:

{
  "prices": [2.5, 4, 6.5]
}

The CEL only tab contains the values and the rule in one expression. The enabled cel.bind() extension keeps each name local to its final argument.

Evaluation step by step

  1. The first price becomes 2.5 * 2.0 = 5.0.
  2. The same transformation runs for the remaining prices.
  3. The result is [5.0, 8.0, 13.0].

Your task

  1. Run the example.
  2. Change the multiplier to 1.2.
  3. Replace the transform with price >= 4.0 and inspect the list of booleans.

Expected observation: A transform may return any supported type; the final experiment returns one boolean per input price.

Common mistake

Do not expect map() with a boolean expression to remove false items. It returns those boolean values.

Show the explanation

Yes. Each output position corresponds to the same input position.

Knowledge check

Does map() preserve input order?

Key takeaway

Use map() to transform every item and filter() to select items.

Sources

  • CEL-DEV — official CEL overview.
  • CEL-LANG — official CEL language definition.
  • CEL-GO-BIND — official documentation for the optional cel.bind() extension used by the self-contained tab.