ADR 11: Keep the last good implementation after a compile failure

Date: 2026-06-16

Context

Re-evaluating a defnz at the REPL can fail to compile. The running system has to decide what happens to the previously bound function.

Decision

If re-evaluating defnz fails to compile, preserve the previous working function when one exists.

Consequences

A failed experiment does not break the running system, which matches the exploratory REPL workflow. The failure surfaces as a diagnostic while the last good binding stays callable.

Alternatives

Unbinding the Var on failure was considered; it destroys working state mid-exploration for no benefit.