Static config files (JSON, YAML) can only hold data. A Tengo config file can compute values, branch on environment, and derive one setting from another — all without recompiling the host application.
fmt := import("fmt")
os := import("os")env := os.getenv("APP_ENV")
if env == "" { env = "development" }config := {
env: env,
debug: true,
log_level: "debug",
db_host: "localhost",
db_port: 5432,
db_pool: 3,
cache_ttl: 60,
rate_limit: 50,
feature_dark_mode: false,
}if env == "production" {
config.debug = false
config.log_level = "warn"
config.db_host = "db.prod.internal"
config.db_pool = 20
config.cache_ttl = 3600
config.rate_limit = 2000
config.feature_dark_mode = true
} else if env == "staging" {
config.log_level = "info"
config.db_host = "db.staging.internal"
config.db_pool = 8
config.cache_ttl = 300
}config.db_url = fmt.sprintf("postgres://%s:%d/app", config.db_host, config.db_port)
config.workers = config.db_pool * 2string_keys := ["env", "log_level", "db_url"]
other_keys := ["debug", "workers", "cache_ttl", "rate_limit", "feature_dark_mode"]
for _, k in string_keys {
fmt.printf("%-20s %s\n", k, config[k])
}
for _, k in other_keys {
fmt.printf("%-20s %v\n", k, config[k])
}try it
env development log_level debug db_url postgres://localhost:5432/app debug true workers 6 cache_ttl 60 rate_limit 50 feature_dark_mode false