Aether in Aether

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Aether's own syntax, written as an Aether grammar. Ichor doesn't read .aether files with this grammar — Aether.Lexer and Aether.Reader are hand-written — so this is a description, kept honest by a test rather than by being the implementation: test/aether/grammar_doc_test.exs compiles the block below and checks that it accepts exactly the files Aether.Reader accepts, across every .aether file in this repository and a set of deliberately broken ones.

See the reference for what each construct means; this page only says what is well-formed.

The grammar

@grammar "aether"
@root grammar_file
@skip TRIVIA

COMMENT := ";" (!"\n" .)*
TRIVIA  := (SPACE | COMMENT)*

UPPER_NAME := [A-Z_] [A-Z0-9_]*
LOWER_NAME := [a-z] [a-z0-9_\-]*
NUMBER     := DIGIT+

ESCAPE     := "\\" ("x" HEX HEX | "u{" HEX+ "}" | [nrt] | [^a-zA-Z0-9])
STRING     := "\"" (ESCAPE | !"\"" .)* "\"" (("cs" | "i") ![a-zA-Z0-9_] | !"cs" !"i")
POSIX      := "[:" ("alpha" | "alnum" | "digit" | "space" | "hex") ":]"
CHAR_CLASS := "[" "^"? (POSIX | ESCAPE | !"]" .)* "]"
REGEX      := "/" ("\\" . | "[" ("\\" . | !"]" .)* "]" | ![/\[] .)* "/"

grammar_file := TRIVIA? header definition* TRIVIA?

header := "@grammar" name:STRING "@root" root:LOWER_NAME pragma*
pragma := "@skip" UPPER_NAME | "@noskip" | "@case_insensitive" | "@engine" LOWER_NAME

definition   := token_def | rule_def | keywords_def
token_def    := UPPER_NAME ":=" choice refine?
rule_def     := LOWER_NAME ":=" choice
keywords_def := "@keywords" UPPER_NAME "{" keyword ("," keyword)* "}"
keyword      := STRING "->" UPPER_NAME
refine       := "@refine" "(" STRING "," STRING ("," UPPER_NAME)* ")"

choice     := sequence ("|" sequence)*
sequence   := (!def_head item)+
def_head   := (UPPER_NAME | LOWER_NAME) ":="
item       := "~"? term
term       := (LOWER_NAME ":")? postfix
postfix    := ("&" | "!") primary | primary quantifier?
quantifier := "*" | "+" | "?" | "{" NUMBER ("," NUMBER?)? "}"
primary    := STRING | CHAR_CLASS | REGEX | "." | UPPER_NAME | LOWER_NAME
            | "(" choice ")" | layout | native

layout     := ("@indent" | "@samecol") ("(" choice ")" | postfix)
native     := "@native" "(" STRING "," STRING ("," LOWER_NAME)* ")" hint?
hint       := "@hint" "(" hint_entry ("," hint_entry)* ")"
hint_entry := LOWER_NAME ":" (LOWER_NAME | "(" (LOWER_NAME ("," LOWER_NAME)*)? ")")

Reading it

  • Upper-case names are tokens, lower-case names are rules — the same split the grammar describes. UPPER_NAME and LOWER_NAME are exactly Aether.Lexer's two identifier shapes; anything else, like Foo, matches neither and fails to lex.
  • sequence stops in front of NAME :=. That lookahead is the whole reason a definition needs no terminator: a bare name is otherwise a perfectly good next term.
  • TRIVIA? at both ends of grammar_file because @skip only splices between the parts of a rule, never before the first or after the last — a file may begin with a comment and end with a newline.
  • Pragmas are header-only. pragma* sits inside header, so a @skip written after the first definition is a syntax error, as it is for Aether.Reader.

Beyond syntax

Aether.Reader checks more than a context-free grammar can say. It also rejects:

  • a character class, . or /regex/ in a rule body, and a rule name in a token body;
  • a name: capture in a token body — a token matches as one piece of text;
  • ~ anywhere but directly before a bare name, inside a token, or under @noskip;
  • a pragma given twice, @skip together with @noskip, and an @engine other than peg, lr or glr;
  • a name declared twice, and both @keywords and @refine on one token;
  • in @hint, keys other than nullable and leading, leading: in a token, and a leading: name that isn't a dependency.

Undefined references, redeclared predefined tokens and left recursion are checked later, by Aether.Eval and Grammar.Analysis, once the whole grammar has been read.