Explanation

Where each value came from

How aless knows the line and column of every value without any grammar recording it, where that is exact, and where it is a best effort.

Every entry aless prints has a line and a col, the status bar shows them for the focused node, the source view opens at them, and a reload falls back on them. None of the grammars records them. aless works them out after the parse.

Matching the tokens to the tree #

The parsing engine reports every token it reads, with the token's position, as it reads it. The parse also produces the tree. aless walks the two side by side: the keys and the leaf values of the tree, in document order, against the tokens that carry a value, in source order. Where a key or a value matches the next such token, it takes that token's position. The search looks a bounded distance ahead, so a token the tree has no node for does not throw the alignment off.

The benefit is that position is a property of every format at once, including a grammar written on the command line, rather than a feature each grammar has to remember to build.

Exact, and best effort #

The alignment is exact wherever the tree's values are the tokens' values: the JSON family, TOML, INI, CSV and TSV, and ZON. It is a best effort where a grammar builds values the source does not spell out. Markdown's syntax tree, XML's element records and parts of YAML make nodes whose text is not one token's text, and a value assembled from several tokens matches none of them.

A node the alignment cannot place has no position, and says so: its line and col are null. It does not inherit its parent's, since a wrong position is worse than none for a caller that jumps to it.

The other direction #

--where --at LINE:COL asks the opposite question, which node a position is in. aless answers with the node that starts last at or before the position on its line, the innermost of several starting there, so a column inside a long value names that value. A line with no node of its own, a comment or a closing bracket, answers with the last node before it. A position outside the text, past the last line or the end of a line, names nothing, and the error says so.

Those rules are what let a linter's or a test's 42:7 become a path: Ask a file questions from a script shows one in use.