Array Comprehensions

Array comprehensions build arrays from other collections using a FOR clause inside a literal【F:language/src/test/kotlin/v2/ebnf.txt†L122-L124】. They’re handy for concise reshaping without an explicit loop.

Basic comprehension

LET nums = [1, 2, 3];
LET squares = [FOR (n IN nums) => n * n];
OUTPUT { squares: squares };

squares becomes [1, 4, 9].

With filtering

LET nums = [1, 2, 3, 4];
LET evens = [FOR (n IN nums) IF n % 2 == 0 => n];
OUTPUT { evens: evens };

evens becomes [2, 4].

Building objects inline

LET items = [
  { name: "pen", qty: 2, price: 9.5 },
  { name: "pad", qty: 1, price: 12.0 },
  { name: "bag", qty: 3, price: 4.75 }
];

LET summaries = [
  FOR (item IN items)
    IF item.qty > 1
    => { name: item.name, subtotal: item.qty * item.price }
];

OUTPUT { summaries: summaries };

Result:

{
  "summaries": [
    { "name": "pen", "subtotal": 19 },
    { "name": "bag", "subtotal": 14.25 }
  ]
}

Trailing form

You can trail the FOR EACH after the expression:

LET nums = [1, 2, 3];
LET doubled = [n * 2 FOR EACH n IN nums];

This is equivalent to the basic form above.

Try it

Paste any of the snippets into the playground and run. Start from the collection-transforms example to compare comprehensions with MAP/FILTER/REDUCE.