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Pos

pub(all) struct Pos {
  line : Int
  pos : Int
  idx : Int
  col0 : Int
  col : @marianoguerra/shrubbery/column.Column
Column
} derive(Eq)

A position in a source file.

Four numbers, in three different units, and keeping them apart is the first thing this port has to get right:

line and pos are what Racket's port-next-location reports, in CODE POINTS. They are what raw-srcloc carries, and what the oracle compares, so they cannot be approximated by anything cheaper.
idx is a UTF-16 code-unit index. It exists to slice a MoonBit String and to hand a span to error-report, and it is never reported.
col is the INDENTATION column, which is a partial order rather than a number — see @column.Column. It is not pos minus the line start: tabs and grapheme clusters both make it something else.

fn Pos::to_display(self : Pos) -> String

line:col, 1-based on both, for a message.

pub(all) struct Span {
  start : Pos
  end : Pos
} derive(Eq)

A half-open range of a source file.

fn Span::at(p : Pos) -> Span

An empty span at p, for an error that has no extent — an unexpected end of input, or a missing closer.

fn Span::merge(self : Span, other : Span) -> Span

The smallest span covering both.

fn Span::units(self : Span) -> Int

Length in UTF-16 code units — the unit error-report spans are in.

let start : Pos