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Span

pub(all) struct Span {
  start : Int
  end : Int
} derive(Eq, @moonbitlang/core/debug.Debug
Debug)

A half-open range of the source text, in UTF-16 code units.

Code units rather than code points, and rather than bytes, because that is what a MoonBit String is indexed in and what error-report labels are measured in. Choosing anything else would put a conversion on every slice and every diagnostic; choosing this one puts it nowhere.

This duplicates css/span almost exactly, and it should. Lifting thirty lines into a shared micro-module would put a dependency in the manifest of both, which every consumer of either would then fetch -- and adding one is a design decision rather than a manifest edit. Thirty duplicated lines is the cheaper side of that trade.

There is deliberately no line or column here. A span is a range, and a range is enough to slice the source and to hand to a renderer, which computes line and column from the text itself. Carrying them would mean keeping them correct through every construction, for the benefit of nothing that cannot recompute them.

Span::at

source

fn Span::at(offset : Int) -> Span

The empty span at one offset.

Span::contains

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fn Span::contains(self : Span, other : Span) -> Bool

Whether other lies within this span.

Span::is_empty

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fn Span::is_empty(self : Span) -> Bool

Span::is_nowhere

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fn Span::is_nowhere(self : Span) -> Bool

Whether this is the placeholder for "no source".

Span::len

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

Span::merge

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

The smallest span covering both.

nowhere is absorbing in the useful direction: merging it with a real span gives the real one back, so building a parent's span by folding over children does not get dragged to offset 0 by one synthetic child.

Span::new

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fn Span::new(start : Int, end : Int) -> Span

Span::slice

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fn Span::slice(self : Span, src : String) -> String

The text this span covers, clamped to the string's bounds.

Clamped rather than checked: a span that runs past the end of its source is a bug, but a diagnostic that crashes while rendering another diagnostic is a worse one.

nowhere

source

let nowhere : Span

The empty span at offset 0, for a node that came from no source at all.

A hand-built tree needs some span, and a fabricated one that pointed somewhere real would make a diagnostic lie about where a problem is. This one is recognisable: is_nowhere is what a renderer checks before drawing a caret.