12.6 The Reader

Scheme’s reader is a recursive-descent parser that can be configured through a readtable and various other parameters. This section describes the reader’s parsing when using the default readtable.

Reading from a stream produces one datum. If the result datum is a compound value, then reading the datum typically requires the reader to call itself recursively to read the component data.

The reader can be invoked in either of two modes: read mode, or read-syntax mode. In read-syntax mode, the result is always a syntax object that includes source-location and (initially empty) lexical information wrapped around the sort of datum that read mode would produce. In the case of pairs, vectors, and boxes, the content is also wrapped recursively as a syntax object. Unless specified otherwise, this section describes the reader’s behavior in read mode, and read-syntax mode does the same modulo wrapping the final result.

Reading is defined in terms of Unicode characters; see Ports for information on how a byte stream is converted to a character stream.

12.6.1 Delimiters and Dispatch

Along with whitespace, the following characters are delimiters:

   ( ) [ ] [ ] " , ' ` ;

A delimited sequence that starts with any other character is typically parsed as either a symbol or number, but a few non-delimiter characters play special roles:

• # has a special meaning as an initial character in a delimited sequence; its meaning depends on the characters that follow; see below.

• | starts a subsequence of characters to be included verbatim in the delimited sequence (i.e,. they are never treated as delimiters, and they are not case-folded when case-insensitivity is enabled); the subsequence is terminated by another |, and neither the initial nor terminating | is part of the subsequence.

• \ outside of a | pair causes the folowing character to be included verbatim in a delimited sequence.

More precisely, after skipping whitespace, the reader dispatches based on the next character or characters in the input stream as follows:

	(		starts a pair or list; see Reading Pairs and Lists
	[		starts a pair or list; see Reading Pairs and Lists
	{		starts a pair or list; see Reading Pairs and Lists
	)		matches ( or raises exn:fail:read
	]		matches [ or raises exn:fail:read
	}		matches { or raises exn:fail:read
	"		starts a string; see Reading Strings
	,		starts a quote; see Reading Quotes
	`		starts a quasiquote; see Reading Quotes
	,		starts an [splicing] unquote; see Reading Quotes
	;		starts a line comment; see Reading Comments
	#t or #T		true; see Reading Booleans
	#f or #F		false; see Reading Booleans
	#(		starts a vector; see Reading Vectors
	#[		starts a vector; see Reading Vectors
	#{		starts a vector; see Reading Vectors
	#s(		starts a structure literal; see Reading Structures
	#s[		starts a structure literal; see Reading Structures
	#s{		starts a structure literal; see Reading Structures
	#\		starts a character; see Reading Characters
	#"		starts a byte string; see Reading Strings
	#%		starts a symbol; see Reading Symbols
	#:		starts a keyword; see Reading Keywords
	#&		starts a box; see Reading Boxes
	#|		starts a block comment; see Reading Comments
	#;		starts an S-expression comment; see Reading Comments
	#,		starts a syntax quote; see Reading Quotes
	#!		starts a line comment; see Reading Comments
	#!/		starts a line comment; see Reading Comments
	#!		may start a reader extension; see Reading via an Extension
	#`		starts a syntax quasiquote; see Reading Quotes
	#,		starts an syntax [splicing] unquote; see Reading Quotes
	#~		starts compiled code; see current-compile
	#i or #I		starts a number; see Reading Numbers
	#e or #E		starts a number; see Reading Numbers
	#x or #X		starts a number; see Reading Numbers
	#o or #O		starts a number; see Reading Numbers
	#d or #D		starts a number; see Reading Numbers
	#b or #B		starts a number; see Reading Numbers
	#<<		starts a string; see Reading Strings
	#rx		starts a regular expression; see Reading Regular Expressions
	#px		starts a regular expression; see Reading Regular Expressions
	#ci, #cI, #Ci, or #CI		switches case sensitivity; see Reading Symbols
	#cs, #cS, #Cs, or #CS		switches case sensitivity; see Reading Symbols
	#sx, #sX, #Sx, or #SX		starts a Scheme expression; see Honu Parsing
	#hx		starts a Honu expression; see Honu Parsing
	#hash		starts a hash table; see Reading Hash Tables
	#reader		starts a reader extension use; see Reading via an Extension
	#lang		starts a reader extension use; see Reading via an Extension
	#⟨digit10⟩+(		starts a vector; see Reading Vectors
	#⟨digit10⟩+[		starts a vector; see Reading Vectors
	#⟨digit10⟩+{		starts a vector; see Reading Vectors
	#⟨digit10⟩{1,8}=		binds a graph tag; see Reading Graph Structure
	#⟨digit10⟩{1,8}#		uses a graph tag; see Reading Graph Structure
	otherwise		starts a symbol; see Reading Symbols

12.6.2 Reading Symbols

Symbols in Guide: PLT Scheme introduces the syntax of symbols.

A sequence that does not start with a delimiter or # is parsed as either a symbol or a number (see Reading Numbers), except that . by itself is never parsed as a symbol or character (unless the read-accept-dot parameter is set to #f). A #% also starts a symbol. A successful number parse takes precedence over a symbol parse.

When the read-case-sensitive parameter is set to #f, characters in the sequence that are not quoted by | or \ are first case-normalized. If the reader encounters #ci, #CI, #Ci, or #cI, then it recursively reads the following datum in case-insensitive mode. If the reader encounters #cs, #CS, #Cs, or #cS, then recursively reads the following datum in case-sensitive mode.

Examples:

Apple  reads equal to  (string->symbol "Apple")  Ap#ple  reads equal to  (string->symbol "Ap#ple")  Ap ple  reads equal to  (string->symbol "Ap")  Ap| |ple  reads equal to  (string->symbol "Ap ple")  Ap\ ple  reads equal to  (string->symbol "Ap ple")  #ci Apple  reads equal to  (string->symbol "apple")  #ci |A|pple  reads equal to  (string->symbol "Apple")  #ci \Apple  reads equal to  (string->symbol "Apple")  #ci#cs Apple  reads equal to  (string->symbol "Apple")  #%Apple  reads equal to  (string->symbol "#%Apple")

12.6.3 Reading Numbers

Numbers in Guide: PLT Scheme introduces the syntax of numbers.

A sequence that does not start with a delimiter is parsed as a number when it matches the following grammar case-insenstively for ⟨number₁₀⟩ (decimal), where n is a meta-meta-variable in the grammar.

A number is optionally prefixed by an exactness specifier, #e (exact) or #i (inexact), which specifies its parsing as an exact or inexact number; see Numbers for information on number exactness. As the non-terminal names suggest, a number that has no exactness specifier and matches only ⟨inexact-number_n⟩ is normally parsed as an inexact number, otherwise it is parsed as an excat number. If the read-decimal-as-inexact parameter is set to #f, then all numbers without an exactness specifier are instead parsed as exact.

If the reader encounters #b (binary), #o (octal), #d (decimal), or #x (hexadecimal), it must be followed by a sequence that is terminated by a delimiter or end-of-file, and that matches the ⟨general-number₂⟩, ⟨general-number₈⟩, ⟨general-number₁₀⟩, or ⟨general-number₁₆⟩ grammar, respectively.

An ⟨exponent-mark_n⟩ in an inexact number serves both to specify an exponent and specify a numerical precision. If single-precision IEEE floating point is supported (see Numbers), the marks f and s specifies single-precision. Otherwise, or with any other mark, double-precision IEEE floating point is used.

	⟨numbern⟩	::=	⟨exactn⟩  |  ⟨inexactn⟩
	⟨exactn⟩	::=	⟨exact-integern⟩  |  ⟨exact-rationaln⟩
		|	⟨exact-complexn⟩
	⟨exact-integern⟩	::=	[⟨sign⟩] ⟨digitsn⟩
	⟨digitsn⟩	::=	⟨digitn⟩+
	⟨exact-rationaln⟩	::=	⟨exact-integern⟩ / ⟨unsigned-integern⟩
	⟨exact-complexn⟩	::=	⟨exact-rationaln⟩ ⟨sign⟩ ⟨exact-rationaln⟩ i
	⟨inexactn⟩	::=	⟨inexact-realn⟩  |  ⟨inexact-complexn⟩
	⟨inexact-realn⟩	::=	[⟨sign⟩] ⟨inexact-normaln⟩
		|	⟨sign⟩ ⟨inexact-specialn⟩
	⟨inexact-unsignedn⟩	::=	⟨inexact-normaln⟩  |  ⟨inexact-specialn⟩
	⟨inexact-normaln⟩	::=	⟨inexact-simplen⟩ [⟨exp-markn⟩ [⟨sign⟩] ⟨digits#n⟩]
	⟨inexact-simplen⟩	::=	⟨digits#n⟩ [.] #*
		|	[⟨exact-integern⟩] . ⟨digits#n⟩
		|	⟨digits#n⟩ / ⟨digits#n⟩
	⟨inexact-specialn⟩	::=	inf.0  |  nan.0
	⟨digits#n⟩	::=	⟨digitn⟩+ #*
	⟨inexact-complexn⟩	::=	[⟨inexact-realn⟩] ⟨sign⟩ ⟨inexact-unsignedn⟩ i
		|	⟨inexact-realn⟩ @ ⟨inexact-realn⟩
	⟨sign⟩	::=	+  |  -
	⟨digit16⟩	::=	⟨digit10⟩  |  a  |  b  |  c  |  d  |  e  |  f
	⟨digit10⟩	::=	⟨digit8⟩  |  8  |  9
	⟨digit8⟩	::=	⟨digit2⟩  |  2  |  3  |  4  |  5  |  6  |  7
	⟨digit2⟩	::=	0  |  1
	⟨exp-mark16⟩	::=	s  |  d  |  l
	⟨exp-mark10⟩	::=	⟨exp-mark16⟩  |  e  |  f
	⟨exp-mark8⟩	::=	⟨exp-mark10⟩
	⟨exp-mark2⟩	::=	⟨exp-mark10⟩
	⟨general-numbern⟩	::=	[⟨exactness⟩] ⟨numbern⟩
	⟨exactness⟩	::=	#e  |  #i

Examples:

-1  reads equal to  -1  1/2  reads equal to  (/ 1 2)  1.0  reads equal to  (inexact->exact 1)  1+2i  reads equal to  (make-complex 1 2)  1/2+3/4i  reads equal to  (make-complex (/ 1 2) (/ 3 4))  1.0+3.0e7i  reads equal to  (inexact->exact (make-complex 1 30000000))  2e5  reads equal to  (inexact->exact 200000)  #i5  reads equal to  (inexact->exact 5)  #e2e5  reads equal to  200000  #x2e5  reads equal to  741  #b101  reads equal to  5

12.6.4 Reading Booleans

A #t or #T is the complete input syntax for the boolean constant true, and #f or #F is the complete input syntax for the boolean constant false.

12.6.5 Reading Pairs and Lists

When the reader encounters a (, [, or {, it starts parsing a pair or list; see Pairs and Lists for information on pairs and lists.

To parse the pair or list, the reader recursively reads data until a matching ), ], or } (respectively) is found, and it specially handles a delimited .. Pairs (), [], and {} are treated the same way, so the remainder of this section simply uses “parentheses” to mean any of these pair.

If the reader finds no delimited . among the elements between parentheses, then it produces a list containing the results of the recursive reads.

If the reader finds two data between the matching parentheses that are separated by a delimited ., then it creates a pair. More generally, if it finds two or more data where the last is preceeded by a delimited ., then it constructs nested pairs: the next-to-last element is paired with the last, then the third-to-last is paired with that pair, and so on.

If the reader finds three or more data between the matching parentheses, and if a pair of delimited .s surrounds any other than the first and last elements, the result is a list containing the element surrounded by .s as the first element, followed by the others in the read order. This convention supports a kind of infix notation at the reader level.

In read-syntax mode, the recursive reads for the pair/list elements are themselves in read-syntax mode, so that the result is list or pair of syntax objects that it itself wrapped as a syntax object. If the reader constructs nested pairs because the input included a single delimited ., then only the innermost pair and outtermost pair are wrapped as syntax objects. Whether wrapping a pair or list, if the pair or list was formed with [ and ], then a 'paren-shape property is attached to the result with the value #\[;if the list or pair was formed with { and }, then a 'paren-shape property is attached to the result with the value #\{.

If a delimited . appears in any other configuration, then the exn:fail:read exception is raised. Similarly, if the reader encounters a ), ], or } that does not end a list being parsed, then the exn:fail:read exception is raised.

Examples:

()  reads equal to  (list)  (1 2 3)  reads equal to  (list 1 2 3)  {1 2 3}  reads equal to  (list 1 2 3)  [1 2 3]  reads equal to  (list 1 2 3)  (1 (2) 3)  reads equal to  (list 1 (list 2) 3)  (1 . 3)  reads equal to  (cons 1 3)  (1 . (3))  reads equal to  (list 1 3)  (1 . 2 . 3)  reads equal to  (list 2 1 3)

If the read-square-bracket-as-paren parameter is set to #f, then when then reader encounters [ and ], the "exn:fail:read" exception is raised. Similarly, If the read-curly-brace-as-paren parameter is set to #f, then when then reader encounters { and }, the "exn:fail:read" exception is raised.

If the read-accept-dot parameter is set to #f, then a delimited . triggers an exn:fail:read exception. If the read-accept-infix-dot parameter is set to #f, then multiple delimited .s trigger an exn:fail:read exception, instead of the infix conversion.

12.6.6 Reading Strings

Strings (Unicode) in Guide: PLT Scheme introduces the syntax of strings.

When the reader encounters ", it begins parsing characters to form a string. The string continues until it is terminated by another " (that is not escaped by \).

Within a string sequence, the following escape sequences are recognized:

• \a: alarm (ASCII 7)

• \b: backspace (ASCII 8)

• \t: tab (ASCII 9)

• \n: linefeed (ASCII 10)

• \v: vertical tab (ASCII 11)

• \f: formfeed (ASCII 12)

• \r: return (ASCII 13)

• \e: escape (ASCII 27)

• \": double-quotes (without terminating the string)

• \': quote (i.e., the backslash has no effect)

• \\: backslash (i.e., the second is not an escaping backslash)

• \⟨digit₈⟩^{1,3}: Unicode for the octal number specified by digit₈^{1,3} (i.e., 1 to 3 ⟨digit₈⟩s) where each ⟨digit₈⟩ is 0, 1, 2, 3, 4, 5, 6, or 7. A longer form takes precedence over a shorter form, and the resulting octal number must be between 0 and 255 decimal, otherwise the exn:fail:read exception is raised.

• \x⟨digit₁₆⟩^{1,2}: Unicode for the hexadecimal number specified by ⟨digit₁₆⟩^{1,2}, where each ⟨digit₁₆⟩ is 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, or f (case-insensitive). The longer form takes precedence over the shorter form.

• \u⟨digit₁₆⟩^{1,4}: like \x, but with up to four hexadecimal digits (longer sequences take precedence). The resulting hexadecimal number must be a valid argument to integer->char, otherwise the exn:fail:read exception is raised.

• \U⟨digit₁₆⟩^{1,8}: like \x, but with up to eight hexadecimal digits (longer sequences take precedence). The resulting hexadecimal number must be a valid argument to integer->char, otherwise the exn:fail:read exception is raised.

• \⟨newline⟩: elided, where ⟨newline⟩ is either a linefeed, carriage return, or carriage return–linefeed combination. This convetion allows single-line strings to span multiple lines in the source.

If the reader encounteres any other use of a backslash in a string constant, the exn:fail:read exception is raised.

Bytes and Byte Strings in Guide: PLT Scheme introduces the syntax of byte strings.

A string constant preceded by # is parsed as a byte-string. (That is, #" starts a byte-string literal.) See Byte Strings for information on byte strings. Byte string constants support the same escape sequences as character strings, except \u and \U.

When the reader encounters #<<, it starts parsing a here string. The characters following #<< until a newline character define a terminator for the string. The content of the string includes all characters between the #<< line and a line whose only content is the specified terminator. More precisely, the content of the string starts after a newline following #<<, and it ends before a newline that is followed by the terminator, where the terminator is itself followed by either a newline or end-of-file. No escape sequences are recognized between the starting and terminating lines; all characters are included in the string (and terminator) literally. A return character is not treated as a line separator in this context. If no characters appear between #<< and a newline or end-of-file, or if an end-of-file is encountered before a terminating line, the exn:fail:read exception is raised.

Examples:

"Apple"  reads equal to  "Apple"  "\x41pple"  reads equal to  "Apple"  "\"Apple\""  reads equal to  "\x22Apple\x22"  "\\"  reads equal to  "\x5C"  #"Apple"  reads equal to  (bytes 65 112 112 108 101)

12.6.7 Reading Quotes

When the reader enounters ', then it recursively reads one datum, and it forms a new list containing the symbol 'quote and the following datum. This convention is mainly useful for reading Scheme code, where 's can be used as a shorthand for (quote s).

Several other sequences are recognized and transformed in a similar way. Longer prefixes take precedence over short ones:

	'	adds	quote
	`	adds	quasiquote
	,	adds	unquote
	,@	adds	unquote-splicing
	#'	adds	syntax
	#`	adds	quasisyntax
	#,	adds	unsyntax
	#,@	adds	unsyntax-splicing

Examples:

'apple  reads equal to  (list 'quote 'apple)  `(1 ,2)  reads equal to  (list 'quasiquote (list 1 (list 'unquote 2)))

The `, ,, and ,@ forms are disabled when the read-accept-quasiquote parameter is set to #f, in which case the exn:fail:read exception is raised, instead.

12.6.8 Reading Comments

A ; starts a line comment. When the reader encounters ;, then it skips past all characters until the next linefeed (ASCII 10), carriage return (ASCII 13), next-line (Unicode 133), line-separator (Unicode 8232), or line-separator (Uunicode 8232) character.

A #| starts a nestable block comment. When the reader encounters #|, then it skips past all characters until a closing |#. Pairs of matching #| and |# can be nested.

A #; starts an S-expression comment. Then the reader encounters #;, it recursively reads one datum, and then discards the datum (continuing on to the next datum for the read result).

A #!  (which is #! followed by a space) or #!/ starts a line comment that can be continued to the next line by ending a line with \. This form of comment normally appears at the beginning of a Unix script file.

Examples:

; comment  reads equal to  nothing  #| a |# 1  reads equal to  1  #| #| a |# 1 |# 2  reads equal to  2  #;1 2  reads equal to  2  #!/bin/sh  reads equal to  nothing  #! /bin/sh  reads equal to  nothing

12.6.9 Reading Vectors

When the reader encounters a #(, #[, or #{, it starts parsing a vector; see Vectors for information on vectors. The #[ and #{ forms can be disabled through the read-square-bracket-as-paren and read-curly-brace-as-paren parameters.

The elements of the vector are recursively read until a matching ), ], or } is found, just as for lists (see Reading Pairs and Lists). A delimited . is not allowed among the vector elements.

An optional vector length can be specified between the # and (, [, or {. The size is specified using a sequence of decimal digits, and the number of elements provided for the vector must be no more than the specified size. If fewer elements are provided, the last provided element is used for the remaining vector slots; if no elements are provided, then 0 is used for all slots.

In read-syntax mode, each recursive read for the vector elements is also in read-syntax mode, so that the wrapped vector’s elements are also wraped as syntax objects, and the vector is immutable.

Examples:

#(1 apple 3)  reads equal to  (vector 1 'apple 3)  #3("apple" "banana")  reads equal to  (vector "apple" "banana" "banana")  #3()  reads equal to  (vector 0 0 0)

12.6.10 Reading Structures

When the reader encounters a #s(, #s[, or #s{, it starts parsing an instance of a prefab structure type; see Structures for information on structure types. The #s[ and #s{ forms can be disabled through the read-square-bracket-as-paren and read-curly-brace-as-paren parameters.

The elements of the structure are recursively read until a matching ), ], or } is found, just as for lists (see Reading Pairs and Lists). A delimited . is not allowed among the elements.

The first element is used as the structure descriptor, and it must have the form (when quoted) of a possible argument to make-prefab-struct; in the simplest case, it can be a symbol. The remaining elements correspond to field values within the structure.

In read-syntax mode, the structure type must not have any mutable fields. The structure’s elements are read in read-syntax mode, so that the wrapped structure’s elements are also wraped as syntax objects.

If the first structure element is not a valid prefab structure type key, or if the number of provided fields is inconsistent with the indicated prefab structure type, the exn:fail:read exception is raised.

12.6.11 Reading Hash Tables

A #hash starts an immutable hash-table constant with key matching based on equal?. The characters after hash must parse as a list of pairs (see Reading Pairs and Lists) with a specific use of delimited .: it must appear between the elements of each pair in the list, and nowhere in the sequence of list elements. The first element of each pair is used as the key for a table entry, and the second element of each pair is the associated value.

A #hasheq starts a hash table like #hash, except that it constructs a hash table based on eq? instead of equal?.

In either case, the table is constructed by adding each mapping to the hash table from left to right, so later mappings can hide earlier mappings if the keys are equivalent.

Examples, where make-... stands for make-immutable-hash:

#hash()  reads equal to  (make-... '())  #hasheq()  reads equal to  (make-...eq '())  #hash(("a" . 5))  reads equal to  (make-... '(("a" . 5)))  #hasheq((a . 5) (b . 7))  reads equal to  (make-...eq '((a . 5) (b . 7)))  #hasheq((a . 5) (a . 7))  reads equal to  (make-...eq '((a . 7)))

12.6.12 Reading Boxes

When the reader encounters a #&, it starts parsing a box; see Boxes for information on boxes. The content of the box is determined by recursively reading the next datum.

In read-syntax mode, the recursive read for the box content is also in read-syntax mode, so that the wrapped box’s content is also wraped as a syntax object, and the box is immutable.

Examples:

#&17  reads equal to  (box 17)

12.6.13 Reading Characters

Characters in Guide: PLT Scheme introduces the syntax of characters.

A #\ starts a character constant, which has one of the following forms:

• #\nul or #\null: NUL (ASCII 0); the next character must not be alphabetic.

• #\backspace: backspace (ASCII 8); the next character must not be alphabetic.

• #\tab: tab (ASCII 9); the next character must not be alphabetic.

• #\newline or #\linefeed: linefeed (ASCII 10); the next character must not be alphabetic.

• #\vtab: vertical tab (ASCII 11); the next character must not be alphabetic.

• #\page: page break (ASCII 12); the next character must not be alphabetic.

• #\return: carriage return (ASCII 13); the next character must not be alphabetic.

• #\space: space (ASCII 32); the next character must not be alphabetic.

• #\rubout: delete (ASCII 127); the next character must not be alphabetic.

• #\⟨digit₈⟩^{1,3}: Unicode for the octal number specified by ⟨digit₈⟩^{1,3}, as in string escapes (see Reading Strings).

• #\x⟨digit₁₆⟩^{1,2}: Unicode for the hexadecimal number specified by ⟨digit₁₆⟩^{1,2}, as in string escapes (see Reading Strings).

• #\u⟨digit₁₆⟩^{1,4}: like #\x, but with up to four hexadecimal digits.

• #\U⟨digit₁₆⟩^{1,6}: like #\x, but with up to six hexadecimal digits.

• #\⟨c⟩: the character ⟨c⟩, as long as #\⟨c⟩ and the characters following it do not match any of the previous cases, and as long as the character after ⟨c⟩ is not alphabetic.

Examples:

#\newline  reads equal to  (integer->char 10)  #\n  reads equal to  (integer->char 110)  #\u3BB  reads equal to  (integer->char 955)  #\λ  reads equal to  (integer->char 955)

12.6.14 Reading Keywords

A #: starts a keyword. The parsing of a keyword after the #: is the same as for a symbol, including case-folding in case-insensitive mode, except that the part after #: is never parsed as a number.

Examples:

#:Apple  reads equal to  (string->keyword "#:Apple")  #:1  reads equal to  (string->keyword "#:1")

12.6.15 Reading Regular Expressions

A #rx or #px starts a regular expression. The characters immediately after #rx or #px must parse as a string or byte string (see Reading Strings). A #rx prefix starts a regular expression as would be constructed by regexp, #px as constructed by pregexp, #rx# as constructed by byte-regexp, and #px# as constructed by byte-pregexp.

Examples:

#rx".*"  reads equal to  (regexp ".*")  #px"[\\s]*"  reads equal to  (pregexp "[\\s]*")  #rx#".*"  reads equal to  (byte-regexp #".*")  #px#"[\\s]*"  reads equal to  (byte-pregexp #"[\\s]*")

12.6.16 Reading Graph Structure

A #⟨digit₁₀⟩^{1,8}= tags the following datum for reference via #⟨digit₁₀⟩^{1,8}#, which allows the reader to produce a datum that have graph structure.

For a specific ⟨digit₁₀⟩^{1,8} in a single read result, each #⟨digit₁₀⟩^{1,8}# reference is replaced by the datum read for the corresponding #⟨digit₁₀⟩^{1,8}=; the definition #⟨digit₁₀⟩^{1,8}= also produces just the datum after it. A #⟨digit₁₀⟩^{1,8}= definition can appear at most once, and a #⟨digit₁₀⟩^{1,8}= definition must appear before a #⟨digit₁₀⟩^{1,8}# reference appears, otherwise the exn:fail:read exception is raised. If the read-accept-graph parameter is set to #f, then #⟨digit₁₀⟩^{1,8}= or #⟨digit₁₀⟩^{1,8}# triggers a exn:fail:read exception.

Although a comment parsed via #; discards the datum afterward, #⟨digit₁₀⟩^{1,8}= definitions in the discarded datum still can be referenced by other parts of the reader input, as long as both the comment and the reference are grouped together by some other form (i.e., some recursive read); a top-level #; comment neither defines nor uses graph tags for other top-level forms.

Examples:

(#1=100 #1# #1#)  reads equal to  (list 100 100 100)  #0=(1 . #0#)  reads equal to  (let ([v (cons 1 #f)])   (set-cdr! v v) v)

12.6.17 Reading via an Extension

When the reader encounters #reader, then it loads an external reader procedure and applies it to the current input stream.

The reader recursively reads the next datum after #reader, and passes it to the procedure that is the value of the current-reader-guard parameter; the result is used as a module path. The module path is passed to dynamic-require with either 'read or 'read-syntax (depending on whether the reader is in read or read-syntax mode).

The arity of the resulting procedure determines whether it accepts extra source-location information: a read procedure accepts either one argument (an input port) or five, and a read-syntax procedure accepts either two arguments (a name value and an input port) or six. In either case, the four optional arguments are the module path (as a syntax object in read-syntax mode) followed by the line (positive exact integer or #f), column (non-negative exact integer or #f), and position (positive exact integer or #f) of the start of the #reader form. The input port is the one whose stream contained #reader, where the stream position is immediately after the recursively-read module path.

The procedure should produce a datum result. If the result is a syntax object in read mode, then it is converted to a datum using syntax->datum; if the result is not a syntax object in read-syntax mode, then it is converted to one using datum->syntax. See also Reader-Extension Procedures for information on the procedure’s results.

If the read-accept-reader parameter is set to #f, then if the reader encounters #reader, the exn:fail:read exception is raised.

The #lang Shorthand in Guide: PLT Scheme introduces #lang.

The #lang reader form is similar to #reader, but more constrained: the #lang must be followed by a single space (ASCII 32), and then a non-empty sequence of alphanumeric ASCII, +, -, _, and/or / characters terminated by whitespace or an end-of-file. The sequence must not start or end with /. A sequence #lang ⟨name⟩ is equivalent to #reader ⟨name⟩/lang/reader, except that the terminating whitespace (if any) is consumed before the external reading procedure is called.

Finally, #! followed by alphanumeric ASCII, +, -, or _ is a synonym for #lang followed by a space. Use of this synonym is discourage except as needed to construct programs that conform to certain grammars, such as that of R⁶RS [Sperber07].

By convention, #lang normally appears at the beginning of a file, possibly after comment forms, to specify the syntax of a module.

12.6.18 Honu Parsing

See Honu for information on #hx and #sx.