Janet `apply` requires a function/cfunction. When tables/structs are used as lookup maps (like deftype fields, multimethod dispatch tables), they get called via `(get f key)` not `(apply f args)`. The evaluator's default call path checks `(function? f)` before `apply`, falling back to `(get f (first args))` for single-arg table/struct calls.
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Janet structs silently omit entries with nil values: `(struct ;[:x nil :y 1])` → `{:y 1}`. The `:x` key is completely dropped from the struct, not just set to nil. Use `@{}` (mutable table) when map needs nil-valued entries. This caused the `&env` implicit binding to fail — `(put new-bindings "&env" {'ns nil})` created a struct that became empty `{}`, so `(:ns &env)` failed with unknown method. Fix: use `@{}` table for bindings that may contain nil values, or use a non-nil sentinel.