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Language reference

The language reference contains a technical reference for the Slice language.

Slice consists of three sub-languages:

  • A core Slice language for defining types and contracts
  • A preprocessing language for conditional compilation
  • A language for describing definitions with doc comments

Each of these sub-languages are in turn specified by a set of two context-free grammars:

  • Lexical grammars specify how Unicode characters are combined to form basic tokens and
  • syntactic grammars specify how tokens produced by the lexical grammar are combined to form full expressions.

Context-free grammars consist of a set of symbols and productions. Symbols are abstract elements of the language that can represent anything from single characters to entire expressions. Productions are sets of rules that specify how symbols can be combined to produce other symbols.

This language reference defines grammars through their productions. These productions are written using ANTLR's version of Extended Backus-Naur Form.

Productions start with the symbol they produce, followed by a colon. This is then followed by a set of rules separated by vertical bars, and finally a semicolon to finish the production. For productions with multiple rules, each rule is matched independently of any others. Additionally, the order of rules is arbitrary and doesn't imply any level of precedence.

Rules are defined by a sequence of one or more symbols. For a rule to match, all its symbols must match, in the order they're specified.By default a symbol is matched a single time, but this is modified by the following characters:

  • ? : match this symbol 0 or 1 times
  • * : match this symbol 0 or more times
  • + : match this symbol 1 or more times

Additionally, symbols can be grouped into sub-rules using parenthesis: (symbol1+ symbol2)?. These sub-rules function the same as rules: their contents must be matched fully, and in order.

Symbols produced by lexical grammars are written in snake_case and symbols produced by syntactic grammars are written in PascalCase.

We define several convenience symbols that exist only to improve readability and do not necessarily appear within actual implementations. These convenience tokens are written in SCREAMING_SNAKE_CASE and are defined by the following regular expressions:

LETTER: "[a-zA-Z]";
DIGIT: "[0-9]";
ALPHANUMERIC: "[_a-zA-Z0-9]";
CHARACTER: "[^\n]";

Additionally, some rules consist of zero symbols, meaning that they match exactly nothing. For the sake of readability, we explicitly state that they match EMPTY. It is important to note that EMPTY is not a symbol, but a placeholder for the absence of symbols.

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