Note. To experiment with L1, please go to l1-lab.lamdera.app
The goal of this article is to explain how one can implement a fault-tolerant parser for a simple language, which we will call L1. Here are a few sentences in L1:
This [i is [b not] a very good] test.
It would be rendered as "This is not a very good test." The language consists of ordinary text and elements, which have the form
[name body]
where the body is itself an element, e.g., plain text, an element, or a list of elements, e.g.,
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[link NYT https://nytimes.com]
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[link "New York Times" https://nytimes.com]
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[fontRGB 255 0 255 [i This text is in [b magenta]]]
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[image caption:Songbird width:400 placement:left https://birds/warbler.png]
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[math a^2 + b^2 = c^2]
The syntax is thus somewhat like Lisp, where
an elements like [i foo] means "render 'foo' in italic.
A conventional parser reads its input and attempts to convert it to an abstract syntax tree — a data structure that expresses an understanding of the underlying grammar. When such a parser encounters an error, it typically bails out with a more or less comprehensible error message. Such behavior is not suited for interactive language editors, where the text will pass in and out of an error state as the user types away. A fault-tolerant parser should in all cases return an abstract syntax tree that makes sense to the greatest degree possible.