---
id: LANG-21
family: GR-LANG
level: core
profiles: [runtime]
posture: normative-target
added: "1.0"
changed: "1.0"
source: https://flowdrop.io/spec/rules/gr-lang/lang-21
specification: FlowDrop Workflow Specification 1.0-draft
licence: CC BY 4.0
---

# LANG-21 — A failed mapper expression fails the node and names the port

*GR-LANG (Part I) · level: core · profiles: runtime · added in 1.0*

## The rule

> **Normative.** This is the rule.
>
> When the expression for a mapper output fails, the node fails, the failure names the output port whose expression failed, and no partial output map is produced.

## Related rules

- Names: LANG-29
- Referenced by: LANG-29

---

Rule identifiers are permanent and are never renumbered. This specification carries
no implementation status: each implementation publishes its own standing against
these rules. Licensed CC BY 4.0.