bible/kit/patterns/import-export/contract.md

Contract: Import / Export Workflows

Import Workflow

Recommended stages:

1. Upload
2. Preview / Validate
3. Confirm
4. Execute
5. Result summary

Rules:

• Validation preview must be human-readable (table/list), not raw JSON only.
• Warnings and errors should be shown per row and in aggregate summary.
• Confirm step should clearly communicate scope and side effects.

Export Workflow

• User must explicitly choose export scope (selected, filtered, all) when ambiguity exists.
• Export format should be explicit (csv, json, etc.).
• Download response must set:
  • Content-Type: text/csv; charset=utf-8
  • Content-Disposition: attachment; filename="..."

CSV Format Rules (Excel-compatible)

These rules are mandatory whenever CSV is exported for spreadsheet users.

Encoding and BOM

• Write UTF-8 BOM (\xEF\xBB\xBF) as the very first bytes of the response.
• Without BOM, Excel on Windows opens UTF-8 CSV as ANSI and garbles Cyrillic/special characters.

w.Write([]byte{0xEF, 0xBB, 0xBF})

Delimiter

• Use semicolon (;) as the field delimiter, not comma.
• Excel in Russian/European locale uses semicolon as the list separator.
• Comma-delimited files open as a single column in these locales.

Numbers

• Write decimal numbers with a comma as the decimal separator: 1 234,56 — not 1234.56.
• Excel in Russian locale does not recognize period as a decimal separator in numeric cells.
• Format integers and floats explicitly; do not rely on Go's default %v or strconv.FormatFloat.
• Use a thin non-breaking space (\u202F) or regular space as a thousands separator when the value benefits from readability (e.g. prices, quantities > 9999).

// correct
fmt.Sprintf("%.2f", price) // then replace "." -> ","
strings.ReplaceAll(fmt.Sprintf("%.2f", price), ".", ",")

// wrong — produces "1234.56", Excel treats it as text in RU locale
fmt.Sprintf("%.2f", price)

Dates

• Write dates as DD.MM.YYYY — the format Excel in Russian locale parses as a date cell automatically.
• Do not use ISO 8601 (2006-01-02) for user-facing CSV; it is not auto-recognized as a date in RU locale.

Text quoting

• Wrap any field that contains the delimiter (;), a newline, or a double-quote in double quotes.
• Escape embedded double-quotes by doubling them: "".
• Use encoding/csv with csv.Writer and set csv.Writer.Comma = ';'; it handles quoting automatically.

Error Handling

• Import errors should map to clear user-facing messages.
• Export errors after streaming starts must degrade gracefully (human-readable fallback).