Attributes

An attribute is a named value binding inside a block or at module scope. It is the primary way to attach data to a configuration node.

Syntax

name = expression

The name must be a valid identifier: it may contain ASCII letters, digits, and underscores, and must not start with a digit. Hyphens are not allowed in attribute names (they are only permitted in inline block IDs).

// Valid attribute names
host        = "localhost"
port_number = 3000
max_retries = 5
_internal   = true

// Invalid — hyphens are not allowed
// max-retries = 5   // parse error

Duplicate Attributes

Declaring the same attribute name twice within the same block is an error:

server {
  port = 8080
  port = 9090  // error: duplicate attribute "port"
}

Value Types

The right-hand side of an attribute can be any WCL expression.

Literals

string_val  = "hello"
int_val     = 42
float_val   = 3.14
bool_val    = true
null_val    = null

Variable References

let base_port = 8000

service {
  port    = base_port
  alt_port = base_port + 1
}

Arithmetic Expressions

timeout     = 30 * 1000          // milliseconds
buffer_size = 4 * 1024 * 1024    // 4 MiB
half_port   = base_port / 2
remainder   = total % batch_size

Function Calls

name_upper = upper("my-service")
tag_count  = len(tags)
checksum   = sha256(payload)
combined   = concat("prefix-", name)

String Interpolation

greeting = "Hello, ${user_name}!"
url      = "https://${host}:${port}/api/v${version}"

See String Interpolation for full details.

Ternary Expressions

mode    = debug ? "verbose" : "quiet"
timeout = is_production ? 5000 : 30000

Queries

Query expressions select blocks from the current scope and return lists or single values:

all_servers  = query server
prod_servers = query server where env == "production"
first_host   = (query server)[0].host

See Query Engine for the complete query syntax.

Refs

A ref creates a typed reference to another block by its inline ID:

database #primary {
  host = "db1.internal"
  port = 5432
}

connection {
  target = ref(db-primary)
}

Lists and Maps

ports    = [8080, 8081, 8082]
labels   = ["web", "api", "public"]
env_vars = { HOST: "0.0.0.0", PORT: "8080" }

Comparison and Logical Expressions

is_valid   = port > 0 && port < 65536
is_dev     = env == "development" || env == "dev"
is_enabled = !disabled
matches    = name =~ "^api-"

Summary