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      1 @node Lexical conventions
      2 @chapter Lexical conventions
      3 
      4 This section gives an informal account of some of the lexical
      5 conventions used in writing Scheme programs. For a formal syntax of
      6 Scheme, see @ref{Formal syntax}.
      7 
      8 @menu
      9 * Identifiers::
     10 * Whitespace and comments::
     11 * Other notations::
     12 * Datum labels::
     13 @end menu
     14 
     15 @node Identifiers
     16 @section Identifiers
     17 
     18 An identifier is any sequence of letters, digits, and ``extended
     19 identifier characters'' provided that it does not have a prefix which
     20 is a valid number. However, the @code{.} token (a single period) used
     21 in the list syntax is not an identifier.
     22 
     23 All implementations of Scheme must support the following extended
     24 identifier characters:
     25 
     26 @example
     27 ! $ % & * + - . / : < = > ? @ ^ _ ~
     28 @end example
     29 
     30 Alternatively, an identifier can be represented by a sequence of
     31 zero or more characters enclosed within vertical lines (@samp{|}),
     32 analogous to string literals.  Any character, including whitespace
     33 characters, but excluding the backslash and vertical line characters,
     34 can appear verbatim in such an identifier. In addition, characters
     35 can be specified using either an @svar{inline hex escape} or the same
     36 escapes available in strings.
     37 
     38 For example, the identifier @code{|H\x65;llo|} is the same identifier
     39 as @code{Hello}, and in an implementation that supports the
     40 appropriate Unicode character the identifier @code{|\x3BB;|} is the
     41 same as the identifier @theultimate{}. What is more, @code{|\t\t|}
     42 and @code{|\x9;\x9;|} are the same. Note that @code{||} is a valid
     43 identifier that is different from any other identifier.
     44 
     45 Here are some examples of identifiers:
     46 
     47 @example
     48 ...                      +
     49 +soup+                   <=?
     50 ->string                 a34kTMNs
     51 lambda                   list->vector
     52 q                        V17a
     53 |two words|              |two\x20;words|
     54 the-word-recursion-has-many-meanings
     55 @end example
     56 
     57 @xref{Formal syntax} for the formal syntax of identifiers.
     58 
     59 Identifiers have two uses within Scheme programs:
     60 
     61 @itemize
     62 @item
     63 Any identifier can be used as a variable or as a syntactic keyword
     64 (see @ref{Variables syntactic keywords and regions,,
     65 Variables@comma{} syntactic keywords@comma{} and regions}
     66 and @ref{Macros}).
     67 
     68 @item
     69 When an identifier appears as a literal or within a literal (see
     70 @ref{Literal expressions}), it is being used to denote a @define{symbol}
     71 (see @ref{Symbols}).
     72 
     73 @end itemize
     74 
     75 In contrast with earlier revisions of the report [@ref{R5RS}], the syntax
     76 distinguishes between upper and lower case in identifiers and in
     77 characters specified using their names. However, it does not
     78 distinguish between upper and lower case in numbers, nor in
     79 @svar{inline hex escapes} used in the syntax of identifiers,
     80 characters, or strings. None of the identifiers defined in this report
     81 contain upper-case characters, even when they appear to do so as a
     82 result of the English-language convention of capitalizing the first
     83 word of a sentence.
     84 
     85 The following directives give explicit control over case folding.
     86 
     87 @lisp
     88 #!fold-case
     89 #!no-fold-case
     90 @end lisp
     91 
     92 These directives can appear anywhere comments are permitted (see
     93 @ref{Whitespace and comments}) but must be followed by a delimiter.
     94 They are treated as comments, except that they affect the reading of
     95 subsequent data from the same port. The @code{#!fold-case} directive
     96 causes subsequent identifiers and character names to be case-folded as
     97 if by string-foldcase (see @ref{Strings}). It has no effect on
     98 character literals. The @code{#!no-fold-case} directive causes a return
     99 to the default, non-folding behavior.
    100 
    101 @node Whitespace and comments
    102 @section Whitespace and comments
    103 
    104 @define{Whitespace} characters include the space, tab, and newline
    105 characters. (Implementations may provide additional whitespace
    106 characters such as page break.) Whitespace is used for improved
    107 readability and as necessary to separate tokens from each other,
    108 a token being an indivisible lexical unit such as an identifier or
    109 number, but is otherwise insignificant. Whitespace can occur between
    110 any two tokens, but not within a token. Whitespace occurring inside
    111 a string or inside a symbol delimited by vertical lines is significant.
    112 
    113 The lexical syntax includes several comment forms. Comments are
    114 treated exactly like whitespace.
    115 
    116 A semicolon (@samp{;}) indicates the start of a line comment. The
    117 comment continues to the end of the line on which the semicolon
    118 appears.
    119 
    120 @sharpindex{;}
    121 
    122 Another way to indicate a comment is to prefix a @svar{datum}
    123 (cf. @ref{External representations formal}) with @code{#;} and
    124 optional @svar{whitespace}. The comment consists of the comment prefix
    125 @code{#;}, the space, and the @svar{datum} together. This notation
    126 is useful for ``commenting out'' sections of code.
    127 
    128 Block comments are indicated with properly nested @code{#|} and
    129 @code{|#} pairs.
    130 
    131 @lisp
    132 #|
    133    The FACT procedure computes the factorial
    134    of a non-negative integer.
    135 |#
    136 (define fact
    137   (lambda (n)
    138     (if (= n 0)
    139         #;(= n 1)
    140         1        ;Base case: return 1
    141         (* n (fact (- n 1))))))
    142 @end lisp
    143 
    144 @node Other notations
    145 @section Other notations
    146 
    147 For a description of the notations used for numbers, see @ref{Numbers}.
    148 
    149 @table @t
    150 
    151 @item @code{. + -}
    152 These are used in numbers, and can also occur anywhere in an
    153 identifier. A delimited plus or minus sign by itself is also an
    154 identifier. A delimited period (not occurring within a number or
    155 identifier) is used in the notation for pairs (@ref{Pairs and lists}),
    156 and to indicate a rest-parameter in a formal parameter list
    157 (@ref{Procedures}). Note that a sequence of two or more periods is an
    158 identifier.
    159 
    160 @item @code{( )}
    161 Parentheses are used for grouping and to notate lists (@ref{Pairs and
    162 lists}).
    163 
    164 @item @code{'}
    165 The apostrophe (single quote) character is used to indicate literal
    166 data (@ref{Literal expressions}).
    167 
    168 @item @code{`}
    169 The grave accent (backquote) character is used to indicate partly
    170 constant data (@ref{Quasiquotation}).
    171 
    172 @item @code{, ,@@}
    173 The character comma and the sequence comma at-sign are used in
    174 conjunction with quasiquotation (@ref{Quasiquotation}).
    175 
    176 @item @code{"}
    177 The quotation mark character is used to delimit strings
    178 (@ref{Strings}).
    179 
    180 @item @code{\}
    181 Backslash is used in the syntax for character constants
    182 (@ref{Characters}) and as an escape character within string constants
    183 (@ref{Strings}) and identifiers (@ref{Lexical structure}).
    184 
    185 @item @code{[ ] @{ @}}
    186 Left and right square and curly brackets (braces) are reserved for
    187 possible future extensions to the language.
    188 
    189 @item @code{#}
    190 The number sign is used for a variety of purposes depending on the
    191 character that immediately follows it:
    192 
    193 @item @code{#t #f}
    194 These are the boolean constants (@ref{Booleans}), along with the
    195 alternatives @code{#true} and @code{#false}.
    196 
    197 @item @code{#\}
    198 This introduces a character constant (@ref{Characters}).
    199 
    200 @item @code{#(}
    201 This introduces a vector constant (@ref{Vectors}). Vector constants are
    202 terminated by @code{)}.
    203 
    204 @item @code{#u8(}
    205 This introduces a bytevector constant (@ref{Bytevectors}). Bytevector
    206 constants are terminated by @code{)} .
    207 
    208 @item @code{#e #i #b #o #d #x}
    209 These are used in the notation for numbers
    210 (@ref{Syntax of numerical constants}).
    211 
    212 @item @code{#@r{@svar{n}}= #@r{@svar{n}}#}
    213 These are used for labeling and referencing other literal data
    214 (@ref{Datum labels}).
    215 
    216 @end table
    217 
    218 @node Datum labels
    219 @section Datum labels
    220 
    221 @deffn  {lexical syntax} #@svar{n}=@svar{datum}
    222 @deffnx {lexical syntax} #@svar{n}#
    223 
    224 The lexical syntax @code{#}@svar{n}@code{=}@svar{datum} reads the same
    225 as @svar{datum}, but also results in @svar{datum} being labelled by
    226 @svar{n}. It is an error if @svar{n} is not a sequence of digits.
    227 
    228 The lexical syntax @code{#}@svar{n}@code{#} serves as a reference to
    229 some object labelled by @code{#}@svar{n}@code{=}; the result is the
    230 same object as the @code{#}@svar{n}@code{=} (see
    231 @ref{Equivalence predicates}). Together, these syntaxes permit the
    232 notation of structures with shared or circular substructure.
    233 
    234 @lisp
    235 (let ((x (list 'a 'b 'c)))
    236   (set-cdr! (cddr x) x)
    237   x) @result{} #0=(a b c . #0#)
    238 @end lisp
    239 
    240 The scope of a datum label is the portion of the outermost datum in
    241 which it appears that is to the right of the label. Consequently, a
    242 reference @code{#}@svar{n}@code{#} can occur only after a label
    243 @code{#}@svar{n}@code{=}; it is an error to attempt a forward
    244 reference. In addition, it is an error if the reference appears as the
    245 labelled object itself (as in @code{#}@svar{n}@code{=}
    246 @code{#}@svar{n}@code{#}), because the object labelled by
    247 @code{#}@svar{n}@code{=} is not well defined in this case.
    248 
    249 It is an error for a @svar{program} or @svar{library} to include
    250 circular references except in literals. In particular, it is an error
    251 for quasiquote (@ref{Quasiquotation}) to contain them.
    252 
    253 @lisp
    254 #1=(begin (display #\x) #1#) @result{} @r{error}
    255 @end lisp
    256 @end deffn