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The Parser Interface

The parser interface

The Citron-generated code contains a parser class that implements the CitronParser protocol. The name of the parser class is Parser by default, but can be changed.

The CitronParser protocol, defined in CitronParser.swift, implements these two parsing methods:

  1. consume(token:code:)

    This methods makes the parser consume one token. This should be called multiple times to pass a sequence of tokens to the parser. Typically, a separate tokenization stage would generate this sequence of tokens. As the tokens get generated, they can be passed to the parser through this method.

    The first argument, token:, is the semantic value of the token, as seen by the code blocks in the grammar. The type of this argument is the %token_type type specified in the grammar file, available to the parser class as the associated type CitronToken.

    The second argument, code:, is the token code that Citron knows this token by. The Citron-generated parser class contains an enum called CitronTokenCode that lists all the terminals in the grammar. This argument should be a value of that enum.

  2. endParsing()

    This method tells the parser that there are no more tokens to consume, and signifies the end of input.

    This method takes no arguments, and returns the parse result. The parse result is the value returned by the code block of a start rule (a.k.a. root rule) of the grammar. If we’re building a parse tree from the input as illustrated above, the parse result would typically be the completely built parse tree.

    The return type of this method is the %nonterminal_type of the start symbol (a.k.a. root symbol) of the grammar.

Error handling

Both consume(token:code:) and endParsing() are throwing methods. They throw when an input token at a certain position is inconsistent with the grammar, or when the input ends prematurely.

Moreover, we can throw errors from within a rule’s code block and those throws would propagate up to one of the two parsing methods.

The lexer interface

Citron offers a simple lexer that can tokenize an input string.

We give the lexer a series of rules with either string or regular expression patterns. The token data that the lexer should output can be of an arbitrary type, so the lexer is defined as a generic type, with the token data as a type parameter. For each string pattern, we should associate it with the token data that should be output, and for each regular expression pattern, we should provide a closure that returns the token data from the matched string.


A few examples of how Citron is used for parsing can be found in the “examples” folder in the project repository.