Levels

DEBUG (0) < INFO (10) < WARN (20) < ERROR (30) < FATAL (40)

The five built-in severities are plain integer ranks, not a keyword-plus-alist lookup, so filtering is a simple numeric comparison:

+level-debug+   ; 0
+level-info+    ; 10
+level-warn+    ; 20
+level-error+   ; 30
+level-fatal+   ; 40

A logger's minimum threshold defaults to +level-info+. Integer custom levels are also accepted — anything that is not one of the five standard values is rendered numerically instead of by name.

Public operations

• (level-name level) — the canonical name ("DEBUG", "INFO", …), or the integer printed as a string for a custom level.
• (level< a b) — true when level a ranks strictly below level b.
• (level<= a b) — true when level a ranks at or below level b.
• (log-enabled-p logger level) — true when level passes logger's threshold; the check every logging macro performs before doing anything else.

Level-gated evaluation

Every logging macro (log, log-default, log-debug, log-info, log-warn, log-error, log-fatal, the log-default-* family, log-condition, and with-log-span) tests the level before evaluating anything else. A filtered call:

• never evaluates the message form
• never evaluates the field-list forms
• never touches the logger's clock
• never reaches the handler

This means it is safe to pass expensive expressions to a log call that might be filtered out — a log-debug call in a production logger set to +level-info+ costs one integer comparison, nothing more.

(log-debug *logger* (expensive-computation) :detail (build-large-plist))
;; expensive-computation and build-large-plist are never called
;; when *logger*'s level is above DEBUG.

log-debug, log-info, log-warn, log-error, and log-fatal always take the logger as their first argument; use the log-default-* family to log through *default-logger* instead. That logger form — and the level form of log/log-default — is evaluated exactly once regardless of whether the level is enabled, so a form with side effects there is safe but neither necessary nor expected.

emit-log is a function, not a macro: it performs the same level check, but its message and field arguments are evaluated before the call, filtered or not.