Convert YAML to TSV Online

Converting YAML to TSV suits records with free text in them. Descriptions, addresses, commit messages and translated strings often contain commas, and in a tab-separated file they need no quotes. TSV output from a YAML list pastes cleanly into a spreadsheet, and it feeds straight into cut, awk, sort and MySQL LOAD DATA, which expects tabs by default.

The YAML needs to be a sequence of mappings at the top level. Each mapping becomes a line, and the union of all keys becomes the header. A file with several documents separated by --- lines is read in full. A document holding a list adds each of its items as a line, and a document holding one mapping adds one line.

YAML block scalars, the multi-line strings written with | or >, keep their line breaks. A value with a line break is quoted in the TSV so the record stays together. Strict tab-separated readers that do not handle quotes will split those records. If that matters, fold the text to one line in the YAML, or replace line breaks after conversion. Anchors and aliases are expanded, so a value shared with &default and *default appears in full on every row that uses it.