Adder

Working with lwlee2608/adder

Adder reads YAML into Go structs and overlays env vars. It does case-insensitive key matching only — it does not fold snake_case ↔ CamelCase. Every rule below exists to avoid fields silently binding to their zero value.

Rules

1. Prefer YAML keys that already match the lowercased Go field name. Adder looks up strings.ToLower(field.Name), so a field SessionTtl is resolved against the YAML key sessionttl. Both lowercase and camelCase keys work because adder lowercases both sides — camelCase is just easier to read.

   # good — matches field name when lowercased
   auth:
     username: admin
     sessionTtl: 24h
   

   type AuthConfig struct {
       Username   string
       SessionTtl time.Duration  // no tag required
   }
   

   Idiomatic Go vs. tag-free binding is a tension. Go style says initialisms should be all-caps (SessionTTL). But strings.ToLower("SessionTTL") is "sessionttl", which forces either an ugly YAML key (sessionttl: 24h) or a mapstructure tag. Pick your poison: idiomatic Go + tag, or non-idiomatic field name + no tag. This skill leans toward the latter to minimize tags, but either is defensible — be consistent within a project.

   This applies recursively at every nesting level — auth.session.ttl resolves each segment the same way.

2. Prefer not to use snake_case in YAML. A snake_case key like session_ttl will never bind to SessionTTL automatically — adder compares sessionttl to session_ttl and finds no match, so the field silently stays at its zero value (e.g. time.Duration(0), which means "expires immediately" for TTLs). If you must use snake_case for readability, every such field requires an explicit mapstructure tag:

   // only if you insist on snake_case YAML
   SessionTTL time.Duration `mapstructure:"session_ttl"`
   

   Prefer rule 1 over this — fewer tags, fewer ways to forget one.

3. Put defaults in application.yml, not in code. application.yml is the canonical source of default values. Code should not paper over missing config with hard-coded fallbacks (e.g. if cfg.SessionTTL == 0 { cfg.SessionTTL = 24*time.Hour }) — that hides genuine misconfiguration. If a value is required, validate at startup and panic with a clear message.

   # application.yml — defaults live here
   auth:
     sessionTtl: 24h
   

4. Rely on AutomaticEnv() for the standard env var pattern. A keyPath is the dotted path adder uses internally — struct field Auth.SessionTtl becomes keyPath auth.sessionttl. With SetEnvKeyReplacer(strings.NewReplacer(".", "_")) and AutomaticEnv(), adder derives the env var as strings.ToUpper(keyPath) with dots replaced by underscores. Don't add BindEnv calls for env vars that already follow this pattern — they're noise.

   keyPath: openrouter.apikey  →  env: OPENROUTER_APIKEY   (auto, no BindEnv needed)
   keyPath: db.url             →  env: DB_URL              (auto)
   

5. Use BindEnv only when the env var name diverges from the auto pattern. Common reason: an established external env name (e.g. OPENROUTER_API_KEY with an underscore between API and KEY) doesn't match the auto-derived OPENROUTER_APIKEY.

   // Only because the external name is OPENROUTER_API_KEY, not OPENROUTER_APIKEY
   _ = adder.BindEnv("openrouter.apikey", "OPENROUTER_API_KEY")
   
   // Don't write this — DB_URL is already automatic from keyPath db.url
   // _ = adder.BindEnv("db.url", "DB_URL")
   

   If you can rename the env var to match the auto pattern, do that instead and drop the BindEnv.

6. Co-locate config structs with the package they configure. Each package owns its own config type (internal/auth/config.go → auth.Config, internal/db/config.go → db.Config). The cmd-level Config is just composition.

   // cmd/myapp/config.go
   type Config struct {
       Auth auth.Config
       DB   db.Config
   }
   

   Adder binds either way — this is for code organization: package owns its fields, masking tags, and any Enabled()/Validate() helpers, and cmd/<app>/config.go stays short. Config types used only by cmd wiring (e.g. LogConfig) can stay in cmd/<app>/.

Verification procedure

After adding or changing a config field (these checks apply at every nesting level — verify the deepest field, not just the top-level struct):

1. Lowercase-match check — does strings.ToLower(fieldName) equal the YAML key exactly? If not, you need a mapstructure tag, or you should rename the YAML key.
2. Default check — is the default value in application.yml? Run the binary with no env overrides and confirm the field has the expected value. Don't rely on memory — log it, e.g. log.Printf("%+v", cfg).
3. Env override check — if the field is meant to be env-overridable, set the auto-derived env var (UPPER_CASE_WITH_UNDERSCORES) and confirm it overrides the YAML default. Same rule: log the loaded value, don't assume.
4. Zero-value trap — for time.Duration, int, float64, or bool fields, a binding miss looks identical to "configured as 0/false". Always test a non-zero default actually loads.

Common mistakes to watch for

• Adding a multi-word field without a mapstructure tag and using snake_case YAML. This is the #1 footgun: the field silently binds to zero. SessionTTL + session_ttl: → 0, not 24h.
• Adding BindEnv for env vars that already follow the auto pattern. Redundant and obscures which bindings are actually needed.
• Hiding misconfiguration with code defaults. A if cfg.X == 0 { cfg.X = default } masks the real bug (the YAML key didn't bind). Fix the binding instead.
• Assuming case-insensitive means snake-aware. It doesn't — caseInsensitiveLookup only normalizes letter case, not separators.
• Forgetting that time.Duration is int64. A missing duration looks like 0s, which most code treats as "no timeout" or "expired" — almost always wrong.