# macro/compile-when Compile-time conditional: evaluate `cond` during compilation and emit `body` only if truthy. If falsy, `body` is skipped entirely — it produces zero runtime code (a single `push_null`), not even a branch. 
[macro/compile-when cond body]
## How it works `cond` is compiled and executed through the VM at compile time (the same mechanism `require` uses to run module-level forms). The result is checked for truthiness: - truthy → `body` is compiled in place, as if the `macro/compile-when` wrapper weren't there. - falsy → `body` is discarded; the call site emits `push_null`. This is the foundation of zero-cost configurable logging: the level check happens during compilation, so filtered-out calls never enter the bytecode. ## Example — level-filtered logging 
def log-level ([os/env "DEFT_LOG_LEVEL"] or "debug")

defmacro debug {|@args|
    [macro/compile-when [in? @{"debug"} $log-level]
        [log/debug [macro/splice $args]]]
}
When this module is `require`d (which runs `def log-level ...` at compile time), each `[debug "msg"]` call site checks the level during compilation. If `DEFT_LOG_LEVEL=error`, the entire `log/debug` call is elided — the bytecode for that call site is a single `push_null`. ## Conditions must be evaluable at compile time `cond` runs through the VM during compilation, so it can reference: - literals (`true`, `false`, `nil`, numbers, strings, keywords) - globals defined before the `macro/compile-when` form (including those defined by `require`d modules, since `require` runs `def` forms at compile time) - pure functions of the above (`in?`, predicates, infix operators inside `(...)`, …) It cannot reference variables that only exist at runtime (locals inside a function body, closure captures, etc.) — those aren't available when the compiler evaluates the condition. ## Notes - If the condition fails to compile or the VM isn't available, `macro/compile-when` conservatively emits `body` (fail-open). - `macro/compile-when` is a special form handled by the compiler, not a macro — it can't be redefined or shadowed. - The condition is evaluated once per call site, at compile time. The level can't change at runtime; changing `DEFT_LOG_LEVEL` requires a recompile. ## See Also `defmacro`, `require`, `if`
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