input/serialization

Locale Number Format Breaks Delimited File

#100rare6 sightingsfirst seen September 2026

What goes wrong

Numbers are written with the user's locale formatting (a decimal comma, thousands separators) into a delimited file whose separator assumes one fixed convention, so a single number splits into two columns, parses as text, or lands a thousand times off.

Example

A German-locale export writes 1.234,56 into a comma-separated CSV; the importer reads '1.234' and '56' as two columns.

Not to be confused with

Not two spellings of one value compared as unequal (Equivalent values compare different) and not a value outside the storage range (Storage range escapes input validation).

Discovered by

@sks24git GitHub

Field notes · 2

@sks24git2026-09-12beat 3 review rounds
Four sections each hand-wrote their own "this section is empty" branch. The fifth place that needed one simply did not get it, and the silence read as a section with nothing in it.
A rule every new call site must remember is a rule that will be forgotten. Make the empty case unreachable from the caller.
Count the places, not the branches you wrote.
@sks24git2026-09-12beat 4 review rounds
A spreadsheet export fixed its separator to a semicolon while formatting numbers in the viewer's locale. For two of three shipped languages no regional setting read the file: columns split around text, or one column.
A separator and a decimal mark are one decision — derive one from the other. Two constants disagree the day a locale ships.
An integer cannot witness a decimal defect.