Witch of Git - ess/blob - src/span.rs

   1 //! A `Span` represents a location in source file.
   2 //!
   3 //! Spans are useful for reporting error messages. They're used during parsing
   4 //! and are carried around in the `Sexp` type so that errors can be reported at
   5 //! a higher level than the s-expressions, for example reporting semantic errors
   6 //! in a Lisp implementation.
   7
   8 /// Represents a way of talking about a region in the source.
   9 ///
  10 /// All spans begin at a point in the source and contain some (possibly empty)
  11 /// amount of text.
  12 pub trait Span {
  13     /// The `Begin` type represents the beginning point of a span.
  14     type Begin;
  15
  16     /// Moves a `Span` forward by `begin`.
  17     ///
  18     /// useful for moving a relatively positioned span to start at a specific
  19     /// absolute position.
  20     fn offset(&self, begin: Self::Begin) -> Self;
  21     /// Returns the point where a span begins.
  22     fn begin(&self) -> Self::Begin;
  23     /// Combines two spans into a single span that contains both of their
  24     /// contents.
  25     fn union(&self, other: &Self) -> Self;
  26 }
  27
  28 /// `ByteSpan` is a very simple span, and the default for all of the parsers in
  29 /// Ess.
  30 ///
  31 /// It represents a half-open range of bytes, the first element is the first
  32 /// byte in the span and the second element is the first by after the span. This
  33 /// means that `&text[span.0..span.1]` works exactly as we would expect it to.
  34 pub type ByteSpan = (usize, usize);
  35
  36 impl Span for ByteSpan {
  37     type Begin = usize;
  38
  39     fn offset(&self, begin: Self::Begin) -> Self {
  40         (self.0 + begin, self.1 + begin)
  41     }
  42
  43     fn begin(&self) -> Self::Begin {
  44         self.0
  45     }
  46
  47     fn union(&self, other: &Self) -> Self {
  48         use std::cmp::{min, max};
  49         (min(self.0, other.0), max(self.1, other.1))
  50     }
  51 }
  52