windmill-labs/windmill · error

Cannot parse default value of argument

Error message

Cannot parse default value of argument

What it means

windmill-parser-nu throws this when slicing the default-value segment of a Nu parameter (batch.get(d..)) fails during parse_nu_signature. The offsets that mark where the default value should begin don't fit the actual parameter text, so the parser cannot extract a default it believes exists and aborts.

Source

Thrown at backend/parsers/windmill-parser-nu/src/lib.rs:114

                )?),
                None,
            ),
            (Some(t), Some(d)) => {
                if t < d {
                    (
                        batch
                            .get(0..t)
                            .ok_or(anyhow!("Cannot parse argument ident"))?
                            .trim(),
                        Some(parse_type(
                            &batch
                                .get(t..d)
                                .ok_or(anyhow!("Cannot parse type of argument"))?,
                        )?),
                        Some(parse_default(
                            &batch
                                .get(d..)
                                .ok_or(anyhow!("Cannot parse default value of argument"))?,
                            &batches,
                            i,
                            &mut compensate_lookahead,
                        )?),
                    )
                } else {
                    bail!("Parsing error `:` should be before `=`\nit likely means you are trying to set default value to record or table which is not supported at the moment.")
                }
            }
        };

        // Check if it is optional
        let optional = {
            let Some(element) = name.chars().last() else {
                bail!("Internal error, cannot check if argument is optional")
            };
            element == '?'
        };

View on GitHub (pinned to e474e8803c)

Solutions

1. Check each parameter with a default in def main [ ... ] for a well-formed single '= default' expression on one line.
2. Remove duplicate '=' signs or stray tokens around the default value.
3. Keep the whole parameter (name: type = default) on a single line inside the bracket list.
4. If the default is complex, simplify it or move the computation into the function body so the signature stays plain.
5. Report valid-but-rejected Nu syntax to windmill-parser-nu maintainers with the exact snippet.

Example fix

// before
x: int = = 3

// after
x: int = 3

// Ensure every defaulted param has exactly one well-formed '= default'
function validateNuDefaults(code) {
  const m = /def\s+main\s*\[([^\]]*)\]/s.exec(code);
  if (!m) return;
  for (const p of m[1].split(',').map(s => s.trim()).filter(Boolean)) {
    const eq = (p.match(/=/g) || []).length;
    if (eq > 1) throw new Error(`param has multiple '=': ${p}`);
    if (eq === 1 && !/^[\w-]+\s*:\s*\S+\s*=\s*\S/.test(p))
      throw new Error(`malformed default: ${p}`);
  }
}

try {
  const sig = await parseNuSignature(code);
} catch (e) {
  if (String(e.message).includes('Cannot parse default value')) {
    sig = { params: [], error: 'invalid Nu default value' };
  } else throw e;
}

When it happens

Trigger: Calling parse_nu_signature on Nu code where a parameter appears to have a default value (the '=' offset d was detected) but the text from d to end of batch cannot be extracted — typically malformed spacing around '=' or a truncated/default region inside a multiline parameter list.

Common situations: Hand-edited Nu signatures with missing or doubled '=' (e.g. 'x: int = = 3'), defaults written across lines, or copy-pasted signatures with unusual whitespace that the offset splitter misreads.

Related errors

• Parsing error `:` should be before `=` it likely means you a
• Nesting is not supported
• Parsing error
• Cannot parse default value for argument
• ${inferedSchema.error}

AI-assisted analysis of windmill-labs/windmill@e474e8803c (2026-09-03). Data as JSON: /api/errors/2df1a9364d4471b1. Report an issue: GitHub.