docs: add EBNF grammar for the reader syntax (doc/grammar.ebnf)
Specify the surface syntax Jolt's reader accepts as an EBNF grammar — the
syntactic half of the contract (behavioural half is test/spec/). Grounded in
src/jolt/reader.janet and verified against the running reader: whitespace
(comma included), comments, #_ discard, scalars (nil/bool/number incl 0x hex &
floats/string/char incl \uNNNN \oNNN & named/keyword incl ::auto/symbol),
collections, reader macros (quote/syntax-quote/unquote/~@/deref/metadata), and
dispatch (#{}, #(), #', #"regex", #?/#?@, tagged #inst/#uuid). Jolt-vs-Clojure
deviations noted inline (no ratios/radix/BigInt literals; PEG regex limits).
Referenced from README.
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doc/grammar.ebnf
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doc/grammar.ebnf
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(* ===========================================================================
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Jolt reader grammar (EBNF)
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===========================================================================
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This grammar specifies the surface syntax accepted by Jolt's reader
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(src/jolt/reader.janet) — the text that `read`/`parse-string`/`load-string`
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turn into data/forms. It is the syntactic half of Jolt's contract; the
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behavioural half lives in test/spec/. Where Jolt diverges from Clojure the
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difference is called out in a comment.
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Notation (ISO-ish EBNF):
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= definition | alternation
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, concatenation ( ) grouping
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[ x ] optional (0 or 1) { x } repetition (0 or more)
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"x" 'x' terminal (* *) comment
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? x ? informal/char-class terminal
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A source file is a sequence of forms; evaluation is form-by-form.
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=========================================================================== *)
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file = { ws | form } ;
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(* --------------------------------------------------------------------------
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Whitespace, separators and comments
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-------------------------------------------------------------------------- *)
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ws = { whitespace-char | comment | discard } ;
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whitespace-char = " " | "\t" | "\n" | "\r" | "," ; (* comma is whitespace *)
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comment = ";" , { ? any char except "\n" ? } , [ "\n" ] ;
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discard = "#_" , ws , form ; (* read then drop next form *)
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(* --------------------------------------------------------------------------
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Forms
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-------------------------------------------------------------------------- *)
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form = scalar
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| collection
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| reader-macro
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| dispatch ;
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scalar = nil | boolean | number | string | character
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| keyword | symbol ;
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collection = list | vector | map ;
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(* --------------------------------------------------------------------------
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Atoms / scalars
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-------------------------------------------------------------------------- *)
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nil = "nil" ;
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boolean = "true" | "false" ;
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(* Numbers: decimal integer, hexadecimal integer, or decimal float, each with
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an optional leading sign. No ratios, radix (NrXXX), BigInt (N) or BigDecimal
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(M) suffixes, octal, or exponents — Jolt numbers are Janet ints/doubles. *)
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number = [ sign ] , ( hex-int | float | integer ) ;
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sign = "+" | "-" ;
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integer = digit , { digit } ;
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hex-int = "0x" , hex-digit , { hex-digit } ;
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float = digit , { digit } , "." , digit , { digit } ;
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digit = "0".."9" ;
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hex-digit = digit | "a".."f" | "A".."F" ;
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(* Strings: double-quoted with backslash escapes. *)
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string = '"' , { string-char } , '"' ;
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string-char = ? any char except '"' and "\" ? | escape ;
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escape = "\" , ( "n" | "t" | "r" | "\" | '"' | ? other ? ) ;
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(* Characters: \x, a named char, or \uNNNN / \oNNN. *)
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character = "\" , ( char-name | ? any single char ? ) ;
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char-name = "newline" | "space" | "tab" | "return"
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| "formfeed" | "backspace" | "nul"
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| "u" , hex-digit , hex-digit , hex-digit , hex-digit
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| "o" , octal-digit , { octal-digit } ;
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octal-digit = "0".."7" ;
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(* Keywords: :name, :ns/name, or auto-resolved ::name. *)
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keyword = "::" , sym-token (* auto-resolved to current ns *)
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| ":" , sym-token ;
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(* Symbols: a token of symbol-constituent chars not parsed as a number. A "/"
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separates an optional namespace from the name. *)
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symbol = sym-token ;
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sym-token = sym-start , { sym-constituent } ;
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sym-start = sym-constituent - ( digit ) ; (* not a leading digit *)
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sym-constituent = letter | digit
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| "*" | "+" | "!" | "_" | "-" | "?" | "." | "<" | ">"
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| "=" | "&" | "|" | "$" | "%" | "/" ;
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letter = "a".."z" | "A".."Z" ;
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(* --------------------------------------------------------------------------
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Collections
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-------------------------------------------------------------------------- *)
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list = "(" , { ws | form } , ")" ; (* -> a Clojure list *)
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vector = "[" , { ws | form } , "]" ; (* -> a persistent vector *)
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map = "{" , { ws | map-entry } , "}" ;
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map-entry = form , ws , form ; (* an even number of forms *)
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(* --------------------------------------------------------------------------
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Reader macros (prefix sugar). Each expands to a 2-element form
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(op operand), e.g. 'x -> (quote x), @a -> (deref a).
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-------------------------------------------------------------------------- *)
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reader-macro = quote | syntax-quote | unquote | unquote-splice
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| deref | metadata ;
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quote = "'" , form ; (* (quote form) *)
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syntax-quote = "`" , form ; (* (syntax-quote form) *)
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unquote-splice = "~@" , form ; (* (unquote-splicing form) *)
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unquote = "~" , form ; (* (unquote form) *)
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deref = "@" , form ; (* (deref form) *)
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metadata = "^" , meta-form , ws , form ; (* attach metadata to form *)
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meta-form = map | keyword | symbol | string ;
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(* --------------------------------------------------------------------------
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Dispatch forms — introduced by "#".
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-------------------------------------------------------------------------- *)
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dispatch = set
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| anon-fn
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| var-quote
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| regex
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| reader-conditional
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| discard (* #_form — see Whitespace above *)
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| tagged-literal ;
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set = "#{" , { ws | form } , "}" ; (* a persistent set *)
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(* Anonymous function. %, %1.. are positional params; %& is the rest param. *)
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anon-fn = "#(" , { ws | form } , ")" ; (* -> (fn* [..] body) *)
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anon-arg = "%" | "%" , digit , { digit } | "%&" ;
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var-quote = "#'" , symbol ; (* (var symbol) *)
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(* Regex literal -> a Janet PEG-backed regex value.
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Supported: groups, greedy/lazy quantifiers, (?:..), lookahead (?=..)/(?!..),
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alternation, anchors ^ $ \b \B, classes, (?i). NOT: lookbehind,
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backreferences, named groups. *)
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regex = '#"' , { ? any char except '"' ? | "\" , ? any char ? } , '"' ;
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(* Reader conditional. Jolt reads as the :clj platform; #?@ splices a sequence. *)
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reader-conditional
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= "#?@" , "(" , { ws | feature , ws , form } , ")"
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| "#?" , "(" , { ws | feature , ws , form } , ")" ;
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feature = ":clj" | ":cljs" | ":default" | keyword ;
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(* Tagged literal: #tag form. Built-in tags include #inst and #uuid; others
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dispatch to a registered data-reader (else error). *)
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tagged-literal = "#" , symbol , ws , form ;
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