String Interpolation

String interpolation lets you embed expressions directly inside string literals. The interpolated value is converted to a string and spliced into the surrounding text at runtime.

Syntax

Use ${expression} inside a double-quoted string or a heredoc:

name = "api-gateway"
port = 8080

url = "http://${name}:${port}/health"
// → "http://api-gateway:8080/health"

The expression inside ${...} can be any valid WCL expression.

Supported Expression Types

Variable References

let env = "production"
label = "env:${env}"
// → "env:production"

Arithmetic

let base = 8000
addr = "port ${base + 80} is in use"
// → "port 8080 is in use"

Function Calls

name = "my-service"
display = "Service: ${upper(name)}"
// → "Service: MY-SERVICE"

items = ["a", "b", "c"]
summary = "Items: ${join(", ", items)}"
// → "Items: a, b, c"

Ternary Expressions

debug = true
mode = "Running in ${debug ? "debug" : "release"} mode"
// → "Running in debug mode"

Member Access

config {
  version = "2.1.0"
}

banner = "WCL config v${config.version}"
// → "WCL config v2.1.0"

Nested Interpolation

Interpolations can be nested when the inner expression itself contains a string with interpolation:

let prefix = "api"
let version = 2
path = "/v${version}/${prefix}-${to_string(version * 10)}"
// → "/v2/api-20"

Type Coercion in Interpolation

When an interpolated expression evaluates to a non-string type, it is automatically converted:

To embed a list or map in a string, use an explicit conversion function such as join() or to_string().

Escaping

To include a literal ${ in a string without triggering interpolation, escape the dollar sign with a backslash:

template = "Use \${variable} in your templates."
// → "Use ${variable} in your templates."

Interpolation in Heredocs

Standard and indented heredocs support interpolation. Raw heredocs (using single-quoted delimiters) do not.

Standard heredoc with interpolation:

let host = "db.internal"
let port = 5432

dsn = <<EOF
postgresql://${host}:${port}/mydb
EOF

Indented heredoc with interpolation:

let name = "my-service"
let version = "1.0.0"

banner = <<-EOF
  Service: ${name}
  Version: ${version}
  EOF

Raw heredoc (interpolation disabled):

example = <<'EOF'
  Use ${variable} to embed values at runtime.
EOF
// → "  Use ${variable} to embed values at runtime.\n"

Practical Examples

Building URLs

let scheme  = "https"
let host    = "api.example.com"
let version = 2

base_url     = "${scheme}://${host}/v${version}"
health_check = "${base_url}/health"

Log Format Strings

let service = "auth"
let level   = "INFO"

log_prefix = "[${upper(level)}] ${service}:"

Configuration File Paths

let app_name = "my-app"
let env      = "production"

config_path = "/etc/${app_name}/${env}/config.yaml"
log_path    = "/var/log/${app_name}/${env}.log"

Dynamic Labels

let region  = "us-east-1"
let zone    = "a"

availability_zone = "${region}${zone}"       // "us-east-1a"
resource_tag      = "zone:${region}-${zone}" // "zone:us-east-1-a"