@ -231,7 +231,7 @@ We write a @tt{brag} program as an BNF grammar, where patterns can be:
@itemize[
@item{the names of other rules (e.g. @racket[chunk])}
@item{literal and symbolic token names (e.g. @racket[";"], @racket[INTEGER])}
@item{quantified patterns (e.g. @litchar{+} to represent one-or-more repetitions)}
@item{quantified patterns (e.g. @litchar{+} to represent one or more repetitions)}
]
The result of a @tt{brag} program is a module with a @racket[parse] function
that can parse tokens and produce a syntax object as a result.
@ -661,35 +661,41 @@ A program in the @tt{brag} language consists of the language line
@litchar{#lang brag}, followed by a collection of @tech{rule}s and
@tech{line comment}s.
A @deftech{rule} is a sequence consisting of: a @tech{rule identifier}, a colon
@litchar{":"}, and a @tech{pattern}.
A @deftech{rule} is a sequence consisting of: a @tech{rule identifier}, a separator (either @litchar{":"} or @litchar{"::="}), and a @tech{pattern}.
A @deftech{rule identifier} is an @tech{identifier} that is not in upper case.
A @deftech{symbolic token identifier} is an @tech{identifier} that is in upper case.
An @deftech{identifier} is a character sequence of letters, numbers, and
characters in @racket["-.!$%&/<=>?^_~@"]. It must not contain
@litchar{*} or @litchar{+}, as those characters are used to denote
quantification.
An @deftech{identifier} is a sequence of letters, numbers, or
characters in the set @racket["-.!$%&/<=>?^_~@"]. It must not contain
@litchar{*}, @litchar{+}, or @litchar|{{}| and @litchar|{}}|, as those characters are used to denote quantification.
A @deftech{pattern} is one of the following:
@itemize[
@item{an implicit sequence of @tech{pattern}s separated by whitespace}
@item{a terminal: either a literal string or a @tech{symbolic token identifier}.
@item{a @deftech{terminal}: either a literal string or a @tech{symbolic token identifier}.
When used in a pattern, both these terminals will match the same set of inputs. A literal string can match the string itself, or a @racket[token] structure whose type field contains that string (or its symbol form). So @racket["FOO"] would match @racket["FOO"], @racket[(token "FOO" "bar")], or @racket[(token 'FOO "bar")]. A symbolic token identifier can also match the string version of the identifier, or a @racket[token] whose type field is the symbol or string form of the identifier. So @racket[FOO] would also match @racket["FOO"], @racket[(token 'FOO "bar")], or @racket[(token "FOO" "bar")]. (In every case, the value of a token, like @racket["bar"], can be anything, and may or may not be the same as its type.)
When used in a pattern, both kinds of terminals will match the same set of inputs.
Because their underlying meanings are the same, the symbolic token identifier ends up being a notational convenience for readability inside a grammar pattern. Typically, the literal string @racket["FOO"] is used to connote ``match the string @racket["FOO"] exactly'' and the symbolic token identifier @racket[FOO] specially connotes ``match any token of type @racket['FOO]''.
A literal string can match the string itself, or a @racket[token] structure whose type field contains that string (or its symbol form). So @racket["FOO"] in a rule pattern would match the tokens @racket["FOO"], @racket[(token "FOO" "bar")], or @racket[(token 'FOO "bar")].
A symbolic token identifier can also match the string version of the identifier, or a @racket[token] whose type field is the symbol or string form of the identifier. So @racket[FOO] in a rule pattern would @emph{also} match the tokens @racket["FOO"], @racket[(token 'FOO "bar")], or @racket[(token "FOO" "bar")]. (In every case, the value of a token, like @racket["bar"], can be anything, and may or may not be the same as its type.)
Because their underlying meanings are the same, the symbolic token identifier ends up being a notational convenience for readability inside a rule pattern. Typically, the literal string @racket["FOO"] is used to connote ``match the string @racket["FOO"] exactly'' and the symbolic token identifier @racket[FOO] specially connotes ``match a token of type @racket['FOO]''.
You @bold{cannot} use the literal string @racket["error"] as a terminal in a grammar, because it's reserved for @tt{brag}. You can, however, adjust your lexer to package it inside a token structure —say, @racket[(token 'ERROR "error")] — and then use the symbolic token identifier @racket[ERROR] in the grammar to match this token structure.
}
@item{a @tech{rule identifier}}
@item{a @deftech{choice pattern}: a sequence of @tech{pattern}s delimited with @litchar{|} characters.}
@item{a @deftech{quantifed pattern}: a @tech{pattern} followed by either @litchar{*} (``zero or more'') or @litchar{+} (``one or more'')}
@item{a @deftech{quantified pattern}: a @tech{pattern} followed by either @litchar{*} (``zero or more'') or @litchar{+} (``one or more''). Quantification can also be denoted by integers within curly brackets. So @litchar|{{2}}| means ``exactly 2''; @litchar|{{2,5}}| means ``between 2 and 5, inclusive''; @litchar|{{2,}}| means ``2 or more''; and @litchar|{{,5}}| means ``up to 5''.}
@item{an @deftech{optional pattern}: a @tech{pattern} surrounded by @litchar{[} and @litchar{]}}
@item{an explicit sequence: a @tech{pattern} surrounded by @litchar{(} and @litchar{)}}]
A @deftech{line comment} begins with either @litchar{#} or @litchar{;} and
@ -931,7 +937,7 @@ bindings. The most important of these is @racket[parse]:
@item{For terminals, the value of the token.}
@item{For @tech{rule identifier}s: the associated parse value for the rule.}
@item{For @tech{choice pattern}s: the associated parse value for one of the matching subpatterns.}
@item{For @tech{quantifed pattern}s and @tech{optional pattern}s: the corresponding values, spliced into the structure.}
@item{For @tech{quantified pattern}s and @tech{optional pattern}s: the corresponding values, spliced into the structure.}
]
Consequently, it's only the presence of @tech{rule identifier}s in a rule's
@ -1090,44 +1096,45 @@ In addition to the exports shown below, the @racketmodname[brag/support] module
@defform[(:* re ...)]{
Repetition of @racket[re] sequence 0 or more times.}
0 or more occurrences of any @racket[re] pattern.}
@defform[(:+ re ...)]{
Repetition of @racket[re] sequence 1 or more times.}
1 or more occurrences of any @racket[re] pattern.}
@defform[(:? re ...)]{
Zero or one occurrence of @racket[re] sequence.}
0 or 1 occurrence of any @racket[re] pattern.}
@defform[(:= n re ...)]{
Exactly @racket[n] occurrences of @racket[re] sequence, where
Exactly @racket[n] occurrences of any @racket[re] pattern, where
@racket[n] must be a literal exact, non-negative number.}
@defform[(:>= n re ...)]{
At least @racket[n] occurrences of @racket[re] sequence, where
At least @racket[n] occurrences of any @racket[re] pattern, where
@racket[n] must be a literal exact, non-negative number.}
@defform[(:** n m re ...)]{
Between @racket[n] and @racket[m] (inclusive) occurrences of
@racket[re] sequence, where @racket[n] must be a literal exact,
any @racket[re] pattern, where @racket[n] must be a literal exact,
non-negative number, and @racket[m] must be literally either
@racket[#f], @racket[+inf.0], or an exact, non-negative number; a
@racket[#f] value for @racket[m] is the same as @racket[+inf.0].}
@defform[(:or re ...)]{
Same as @racket[(union re ...)].}
Same as @racket[(:union re ...)].}
@deftogether[(
@defform[(:: re ...)]
@defform[(:seq re ...)]
)]{
Both forms concatenate the @racket[re]s.}
Both forms concatenate the @racket[re]s into a single, indivisible pattern.
In other words, this matches @emph{all} the @racket[re]s in order, whereas @racket[(:union re ...)] matches @emph{any} of the @racket[re]s.}
Matthew Butterick
7 years ago
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