Advanced Language Features

Advanced Pkl features are mostly for template and library authors. Reach for them when repeated object structure becomes harder to maintain than the feature itself.

Generators

for generators create elements or entries from an iterable. when generators include members conditionally.

ports = new Listing {  for (offset in List(0, 1, 2)) {    8080 + offset  }}settings {  when (environment == "prod") {    replicas = 3  }}

Spread Syntax

Spread syntax copies members from an iterable or object into the enclosing object. Use nullable spread when the source may be absent.

defaults {  timeout = 30.s}service {  ...defaults  replicas = 2}

Member Predicates

Member predicates amend matching members in a listing, mapping, or dynamic object. They are powerful, but they should read like a query rather than a hidden side effect.

containers {  new { name = "api"; memory = 256.mb }  new { name = "worker"; memory = 256.mb }}tuned = (containers) {  [[name == "api"]] {    memory = 512.mb  }}

Receiver Keywords

Use this for the current receiver, outer for the immediately enclosing object, super for the amended parent, and module for the module receiver or module self type. Prefer local names when nested receiver lookup becomes hard to read.

Type Tests and Casts

is, as, and nullable access help narrow values near the point of use. Keep casts close to the boundary that made the value uncertain.

Glob Patterns

Glob patterns appear in globbed imports and reads. Keep them specific enough that a new file cannot silently change the module graph.