RUNTIME006

stack overflow

error

verifiedverified on v3.7.0 categoryruntime terminalstable code

What it means

Each Mozhi call consumes C stack. Since v3.2.0 the interpreter counts nested calls and stops at the limit with this diagnostic instead of crashing. The stack rlimit is also raised at startup so legitimate deep recursion has headroom.

Trigger condition

A recursion (or mutual recursion) exceeds the call-depth limit — previously this crashed the interpreter with a stack overflow SIGSEGV.

Minimal failing example

fn f(n) { return 1 + f(n) }
echo(string(f(1)))

Verified fix

fn f(n) { if n <= 0 { return 0 } return 1 + f(n-1) }
echo(string(f(500)))

Every recursion needs a terminating condition that eventually holds.

What the interpreter prints

error[RUNTIME006]: stack overflow
  --> app.mz:1:16
 1 | fn f(n) { return 1 + f(n) }
   |                 ^
   |
reason: (the exact problem, with positions)
help:   (a suggested fix appears here)
docs:   https://crossberry-in.github.io/mozhi-doc/errors/RUNTIME006

Run mozhi --json run app.mz to get the same information as machine-readable JSON (schema mozhi.diagnostics.v1), or --verbose to also show grouped related diagnostics. See the CLI reference.

Suggested fix

Add/strengthen the base case, or convert to a loop (`while`). Verified: depth 5000 completes; the default guard fires at 8000 nested calls.

Related syntax

Functions, recursion, while loops

Related diagnostics

None

Reference links

Limitations

The depth limit is fixed (8000) in v3.2.0 and not yet configurable. Extremely deep legitimate recursion must be restructured into loops.