SRFI 14: Character-set Library

# Title

SRFI 14: Character-set Library

# Author

Olin Shivers

# Status

This SRFI is currently in ``final'' status. To see an explanation of each status that a SRFI can hold, see here. You can access the discussion via the archive of the mailing list.

- Received: 1999/10/17
- Draft: 1999/10/30-1999/12/30
- Revised: 2000/04/30
- Revised: 2000/04/30
- Revised: 2000/06/09
- Revised: 2000/12/23

# Table of contents

# Abstract

The ability to efficiently represent and manipulate sets of characters is an unglamorous but very useful capability for text-processing code -- one that tends to pop up in the definitions of other libraries. Hence it is useful to specify a general substrate for this functionality early. This SRFI defines a general library that provides this functionality. It is accompanied by a reference implementation for the spec. The reference implementation is fairly efficient, straightforwardly portable, and has a "free software" copyright. The implementation is tuned for "small" 7 or 8 bit character types, such as ASCII or Latin-1; the data structures and algorithms would have to be altered for larger 16 or 32 bit character types such as Unicode -- however, the specs have been carefully designed with these larger character types in mind. Several forthcoming SRFIs can be defined in terms of this one:

- string library
- delimited input procedures (e.g., `read-line`)
- regular expressions

# Variable Index

Here is the complete set of bindings -- procedural and otherwise -- exported by this library. In a Scheme system that has a module or package system, these procedures should be contained in a module named "char-set-lib".

Predicates & comparison

      char-set? char-set= char-set<= char-set-hash

Iterating over character sets

      char-set-cursor char-set-ref char-set-cursor-next end-of-char-set?
      char-set-fold char-set-unfold char-set-unfold!
      char-set-for-each char-set-map

Creating character sets

      char-set-copy char-set

      list->char-set  string->char-set
      list->char-set! string->char-set!

      char-set-filter  ucs-range->char-set
      char-set-filter! ucs-range->char-set!

      ->char-set

Querying character sets

      char-set->list char-set->string
      char-set-size char-set-count char-set-contains?
      char-set-every char-set-any

Character-set algebra

      char-set-adjoin  char-set-delete
      char-set-adjoin! char-set-delete!

      char-set-complement  char-set-union  char-set-intersection
      char-set-complement! char-set-union! char-set-intersection!

      char-set-difference  char-set-xor  char-set-diff+intersection
      char-set-difference! char-set-xor! char-set-diff+intersection!

Standard character sets

      char-set:lower-case  char-set:upper-case  char-set:title-case
      char-set:letter      char-set:digit       char-set:letter+digit
      char-set:graphic     char-set:printing    char-set:whitespace
      char-set:iso-control char-set:punctuation char-set:symbol
      char-set:hex-digit   char-set:blank       char-set:ascii
      char-set:empty       char-set:full

# Rationale

The ability to efficiently manipulate sets of characters is quite useful for text-processing code. Encapsulating this functionality in a general, efficiently implemented library can assist all such code. This library defines a new data structure to represent these sets, called a "char-set." The char-set type is distinct from all other types.

This library is designed to be portable across implementations that use different character types and representations, especially ASCII, Latin-1 and Unicode. Some effort has been made to preserve compatibility with Java in the Unicode case (see the definition of `char-set:whitespace` for the single real deviation).

## Linear-update operations

The procedures of this SRFI, by default, are "pure functional" -- they do not alter their parameters. However, this SRFI defines a set of "linear-update" procedures which have a hybrid pure-functional/side-effecting semantics: they are allowed, but not required, to side-effect one of their parameters in order to construct their result. An implementation may legally implement these procedures as pure, side-effect-free functions, or it may implement them using side effects, depending upon the details of what is the most efficient or simple to implement in terms of the underlying representation.

The linear-update routines all have names ending with "!".

Clients of these procedures may not rely upon these procedures working by side effect. For example, this is not guaranteed to work:

    (let* ((cs1 (char-set #\a #\b #\c))      ; cs1 = {a,b,c}.
           (cs2 (char-set-adjoin! cs1 #\d))) ; Add d to {a,b,c}.
      cs1) ; Could be either {a,b,c} or {a,b,c,d}.

However, this is well-defined:

    (let ((cs (char-set #\a #\b #\c)))
      (char-set-adjoin! cs #\d)) ; Add d to {a,b,c}.

So clients of these procedures write in a functional style, but must additionally be sure that, when the procedure is called, there are no other live pointers to the potentially-modified character set (hence the term "linear update").

There are two benefits to this convention:

- Implementations are free to provide the most efficient possible implementation, either functional or side-effecting.
- Programmers may nonetheless continue to assume that character sets are purely functional data structures: they may be reliably shared without needing to be copied, uniquified, and so forth.

Note that pure functional representations are the right thing for ASCII- or Latin-1-based Scheme implementations, since a char-set can be represented in an ASCII Scheme with 4 32-bit words. Pure set-algebra operations on such a representation are very fast and efficient. Programmers who code using linear-update operations are guaranteed the system will provide the best implementation across multiple platforms.

In practice, these procedures are most useful for efficiently constructing character sets in a side-effecting manner, in some limited local context, before passing the character set outside the local construction scope to be used in a functional manner.

Scheme provides no assistance in checking the linearity of the potentially side-effected parameters passed to these functions --- there's no linear type checker or run-time mechanism for detecting violations. (But sophisticated programming environments, such as DrScheme, might help.)

## Extra-SRFI recommendations

Users are cautioned that the R5RS predicates

`char-alphabetic?
char-numeric?
char-whitespace?
char-upper-case?
char-lower-case?
`

may or may not be in agreement with the SRFI 14 base character sets

`char-set:letter
char-set:digit
char-set:whitespace
char-set:upper-case
char-set:lower-case
`

Implementors are strongly encouraged to bring these predicates into agreement with the base character sets of this SRFI; not to do so risks major confusion.

# Specification

In the following procedure specifications:

- A cs parameter is a character set.
- An s parameter is a string.
- A char parameter is a character.
- A char-list parameter is a list of characters.
- A pred parameter is a unary character predicate procedure, returning a true/false value when applied to a character.
- An obj parameter may be any value at all.

Passing values to procedures with these parameters that do not satisfy these types is an error.

Unless otherwise noted in the specification of a procedure, procedures always return character sets that are distinct (from the point of view of the linear-update operations) from the parameter character sets. For example, `char-set-adjoin` is guaranteed to provide a fresh character set, even if it is not given any character parameters.

Parameters given in square brackets are optional. Unless otherwise noted in the text describing the procedure, any prefix of these optional parameters may be supplied, from zero arguments to the full list. When a procedure returns multiple values, this is shown by listing the return values in square brackets, as well. So, for example, the procedure with signature

    halts? f [x init-store] -> [boolean integer]

would take one (f), two (f, x) or three (f, x, init-store) input parameters, and return two values, a boolean and an integer.

A parameter followed by "`...`" means zero-or-more elements. So the procedure with the signature

    sum-squares x ...  -> number

takes zero or more arguments (x ...), while the procedure with signature

    spell-check doc dict1 dict2 ... -> string-list

takes two required parameters (doc and dict1) and zero or more optional parameters (dict2 ...).

## General procedures

`char-set?` obj -> boolean
  Is the object obj a character set?
`char-set=` cs1 ... -> boolean
  Are the character sets equal?

  Boundary cases:

      (char-set=) => true
      (char-set= cs) => true

  Rationale: transitive binary relations are generally extended to n-ary relations in Scheme, which enables clearer, more concise code to be written. While the zero-argument and one-argument cases will almost certainly not arise in first-order uses of such relations, they may well arise in higher-order cases or macro-generated code. E.g., consider

      (apply char-set= cset-list)

  This is well-defined if the list is empty or a singleton list. Hence we extend these relations to any number of arguments. Implementors have reported actual uses of n-ary relations in higher-order cases allowing for fewer than two arguments. The way of Scheme is to handle the general case; we provide the fully general extension.

  A counter-argument to this extension is that R5RS's transitive binary arithmetic relations (`=`, `<`, etc.) require at least two arguments, hence this decision is a break with the prior convention -- although it is at least one that is backwards-compatible.

`char-set<=` cs1 ... -> boolean
  Returns true if every character set csi is a subset of character set csi+1.

  Boundary cases:

      (char-set<=) => true
      (char-set<= cs) => true

  Rationale: See `char-set=` for discussion of zero- and one-argument applications. Consider testing a list of char-sets for monotonicity with

      (apply char-set<= cset-list)

`char-set-hash` cs [bound] -> integer
  Compute a hash value for the character set cs. Bound is a non-negative exact integer specifying the range of the hash function. A positive value restricts the return value to the range [0,bound).

  If bound is either zero or not given, the implementation may use an implementation-specific default value, chosen to be as large as is efficiently practical. For instance, the default range might be chosen for a given implementation to map all strings into the range of integers that can be represented with a single machine word.

  Invariant:

      (char-set= cs1 cs2) => (= (char-set-hash cs1 b) (char-set-hash cs2 b))

  A legal but nonetheless discouraged implementation:

      (define (char-set-hash cs . maybe-bound) 1)

  Rationale: allowing the user to specify an explicit bound simplifies user code by removing the mod operation that typically accompanies every hash computation, and also may allow the implementation of the hash function to exploit a reduced range to efficiently compute the hash value. E.g., for small bounds, the hash function may be computed in a fashion such that intermediate values never overflow into bignum integers, allowing the implementor to provide a fixnum-specific "fast path" for computing the common cases very rapidly.

## Iterating over character sets

`char-set-cursor` cset -> cursor
`char-set-ref` cset cursor -> char
`char-set-cursor-next` cset cursor -> cursor
`end-of-char-set?` cursor -> boolean
  Cursors are a low-level facility for iterating over the characters in a set. A cursor is a value that indexes a character in a char set. `char-set-cursor` produces a new cursor for a given char set. The set element indexed by the cursor is fetched with `char-set-ref`. A cursor index is incremented with `char-set-cursor-next`; in this way, code can step through every character in a char set. Stepping a cursor "past the end" of a char set produces a cursor that answers true to `end-of-char-set?`. It is an error to pass such a cursor to `char-set-ref` or to `char-set-cursor-next`.

  A cursor value may not be used in conjunction with a different character set; if it is passed to `char-set-ref` or `char-set-cursor-next` with a character set other than the one used to create it, the results and effects are undefined.

  Cursor values are not necessarily distinct from other types. They may be integers, linked lists, records, procedures or other values. This license is granted to allow cursors to be very "lightweight" values suitable for tight iteration, even in fairly simple implementations.

  Note that these primitives are necessary to export an iteration facility for char sets to loop macros.

  Example:

      (define cs (char-set #\G #\a #\T #\e #\c #\h))

      ;; Collect elts of CS into a list.
      (let lp ((cur (char-set-cursor cs)) (ans '()))
        (if (end-of-char-set? cur) ans
            (lp (char-set-cursor-next cs cur)
                (cons (char-set-ref cs cur) ans))))
        => (#\G #\T #\a #\c #\e #\h)

      ;; Equivalently, using a list unfold (from SRFI 1):
      (unfold-right end-of-char-set?
                    (curry char-set-ref cs)
      	      (curry char-set-cursor-next cs)
      	      (char-set-cursor cs))
        => (#\G #\T #\a #\c #\e #\h)

  Rationale: Note that the cursor API's four functions "fit" the functional protocol used by the unfolders provided by the list, string and char-set SRFIs (see the example above). By way of contrast, here is a simpler, two-function API that was rejected for failing this criterion. Besides `char-set-cursor`, it provided a single function that mapped a cursor and a character set to two values, the indexed character and the next cursor. If the cursor had exhausted the character set, then this function returned false instead of the character value, and another end-of-char-set cursor. In this way, the other three functions of the current API were combined together.

`char-set-fold` kons knil cs -> object
  This is the fundamental iterator for character sets. Applies the function kons across the character set cs using initial state value knil. That is, if cs is the empty set, the procedure returns knil. Otherwise, some element c of cs is chosen; let cs' be the remaining, unchosen characters. The procedure returns

      (char-set-fold kons (kons c knil) cs')

  Examples:

      ;; CHAR-SET-MEMBERS
      (lambda (cs) (char-set-fold cons '() cs))

      ;; CHAR-SET-SIZE
      (lambda (cs) (char-set-fold (lambda (c i) (+ i 1)) 0 cs))

      ;; How many vowels in the char set?
      (lambda (cs)
        (char-set-fold (lambda (c i) (if (vowel? c) (+ i 1) i))
                       0 cs))

`char-set-unfold` f p g seed [base-cs] -> char-set
`char-set-unfold!` f p g seed base-cs -> char-set
  This is a fundamental constructor for char-sets.
  - G is used to generate a series of "seed" values from the initial seed: seed, (g seed), (g2 seed), (g3 seed), ...
  - P tells us when to stop -- when it returns true when applied to one of these seed values.
  - F maps each seed value to a character. These characters are added to the base character set base-cs to form the result; base-cs defaults to the empty set. `char-set-unfold!` adds the characters to base-cs in a linear-update -- it is allowed, but not required, to side-effect and use base-cs's storage to construct the result.

  More precisely, the following definitions hold, ignoring the optional-argument issues:

      (define (char-set-unfold p f g seed base-cs)
        (char-set-unfold! p f g seed (char-set-copy base-cs)))

      (define (char-set-unfold! p f g seed base-cs)
        (let lp ((seed seed) (cs base-cs))
              (if (p seed) cs                                 ; P says we are done.
                  (lp (g seed)                                ; Loop on (G SEED).
                      (char-set-adjoin! cs (f seed))))))      ; Add (F SEED) to set.

  (Note that the actual implementation may be more efficient.)

  Examples:

      (port->char-set p) = (char-set-unfold eof-object? values
                                            (lambda (x) (read-char p))
                                            (read-char p))

      (list->char-set lis) = (char-set-unfold null? car cdr lis)

`char-set-for-each` proc cs -> unspecified
  Apply procedure proc to each character in the character set cs. Note that the order in which proc is applied to the characters in the set is not specified, and may even change from one procedure application to another.

  Nothing at all is specified about the value returned by this procedure; it is not even required to be consistent from call to call. It is simply required to be a value (or values) that may be passed to a command continuation, e.g. as the value of an expression appearing as a non-terminal subform of a `begin` expression. Note that in R5RS, this restricts the procedure to returning a single value; non-R5RS systems may not even provide this restriction.

`char-set-map` proc cs -> char-set
  proc is a char->char procedure. Apply it to all the characters in the char-set cs, and collect the results into a new character set.

  Essentially lifts proc from a char->char procedure to a char-set -> char-set procedure.

  Example:

      (char-set-map char-downcase cset)

## Creating character sets

`char-set-copy` cs -> char-set
  Returns a copy of the character set cs. "Copy" means that if either the input parameter or the result value of this procedure is passed to one of the linear-update procedures described below, the other character set is guaranteed not to be altered.

  A system that provides pure-functional implementations of the linear-operator suite could implement this procedure as the identity function -- so copies are not guaranteed to be distinct by `eq?`.

`char-set` char1 ... -> char-set
  Return a character set containing the given characters.
`list->char-set` char-list [base-cs] -> char-set
`list->char-set!` char-list base-cs -> char-set
  Return a character set containing the characters in the list of characters char-list.

  If character set base-cs is provided, the characters from char-list are added to it. `list->char-set!` is allowed, but not required, to side-effect and reuse the storage in base-cs; `list->char-set` produces a fresh character set.

`string->char-set` s [base-cs] -> char-set
`string->char-set!` s base-cs -> char-set
  Return a character set containing the characters in the string s.

  If character set base-cs is provided, the characters from s are added to it. `string->char-set!` is allowed, but not required, to side-effect and reuse the storage in base-cs; `string->char-set` produces a fresh character set.

`char-set-filter` pred cs [base-cs] -> char-set
`char-set-filter!` pred cs base-cs -> char-set
  Returns a character set containing every character c in cs such that `(pred c)` returns true.

  If character set base-cs is provided, the characters specified by pred are added to it. `char-set-filter!` is allowed, but not required, to side-effect and reuse the storage in base-cs; `char-set-filter` produces a fresh character set.

  An implementation may not save away a reference to pred and invoke it after `char-set-filter` or `char-set-filter!` returns -- that is, "lazy," on-demand implementations are not allowed, as pred may have external dependencies on mutable data or have other side-effects.

  Rationale: This procedure provides a means of converting a character predicate into its equivalent character set; the cs parameter allows the programmer to bound the predicate's domain. Programmers should be aware that filtering a character set such as `char-set:full` could be a very expensive operation in an implementation that provided an extremely large character type, such as 32-bit Unicode. An earlier draft of this library provided a simple `predicate->char-set` procedure, which was rejected in favor of `char-set-filter` for this reason.

`ucs-range->char-set` lower upper [error? base-cs] -> char-set
`ucs-range->char-set!` lower upper error? base-cs -> char-set
  Lower and upper are exact non-negative integers; lower <= upper.

  Returns a character set containing every character whose ISO/IEC 10646 UCS-4 code lies in the half-open range [lower,upper).

  - If the requested range includes unassigned UCS values, these are silently ignored (the current UCS specification has "holes" in the space of assigned codes).
  - If the requested range includes "private" or "user space" codes, these are handled in an implementation-specific manner; however, a UCS- or Unicode-based Scheme implementation should pass them through transparently.
  - If any code from the requested range specifies a valid, assigned UCS character that has no corresponding representative in the implementation's character type, then (1) an error is raised if error? is true, and (2) the code is ignored if error? is false (the default). This might happen, for example, if the implementation uses ASCII characters, and the requested range includes non-ASCII characters.

  If character set base-cs is provided, the characters specified by the range are added to it. `ucs-range->char-set!` is allowed, but not required, to side-effect and reuse the storage in base-cs; `ucs-range->char-set` produces a fresh character set.

  Note that ASCII codes are a subset of the Latin-1 codes, which are in turn a subset of the 16-bit Unicode codes, which are themselves a subset of the 32-bit UCS-4 codes. We commit to a specific encoding in this routine, regardless of the underlying representation of characters, so that client code using this library will be portable. I.e., a conformant Scheme implementation may use EBCDIC or SHIFT-JIS to encode characters; it must simply map the UCS characters from the given range into the native representation when possible, and report errors when not possible.

`->char-set` x -> char-set
  Coerces x into a char-set. X may be a string, character or char-set. A string is converted to the set of its constituent characters; a character is converted to a singleton set; a char-set is returned as-is. This procedure is intended for use by other procedures that want to provide "user-friendly," wide-spectrum interfaces to their clients.

## Querying character sets

`char-set-size` cs -> integer
  Returns the number of elements in character set cs.
`char-set-count` pred cs -> integer
  Apply pred to the chars of character set cs, and return the number of chars that caused the predicate to return true.
`char-set->list` cs -> character-list
  This procedure returns a list of the members of character set cs. The order in which cs's characters appear in the list is not defined, and may be different from one call to another.
`char-set->string` cs -> string
  This procedure returns a string containing the members of character set cs. The order in which cs's characters appear in the string is not defined, and may be different from one call to another.
`char-set-contains?` cs char -> boolean
  This procedure tests char for membership in character set cs.

  The MIT Scheme character-set package called this procedure char-set-member?, but the argument order isn't consistent with the name.

`char-set-every` pred cs -> boolean
`char-set-any` pred cs -> boolean
  The `char-set-every` procedure returns true if predicate pred returns true of every character in the character set cs. Likewise, `char-set-any` applies pred to every character in character set cs, and returns the first true value it finds. If no character produces a true value, it returns false. The order in which these procedures sequence through the elements of cs is not specified.

  Note that if you need to determine the actual character on which a predicate returns true, use `char-set-any` and arrange for the predicate to return the character parameter as its true value, e.g.

      (char-set-any (lambda (c) (and (char-upper-case? c) c))
                    cs)

## Character-set algebra

`char-set-adjoin` cs char1 ... -> char-set
`char-set-delete` cs char1 ... -> char-set
  Add/delete the chari characters to/from character set cs.
`char-set-adjoin!` cs char1 ... -> char-set
`char-set-delete!` cs char1 ... -> char-set
  Linear-update variants. These procedures are allowed, but not required, to side-effect their first parameter.
`char-set-complement` cs -> char-set
`char-set-union` cs1 ... -> char-set
`char-set-intersection` cs1 ... -> char-set
`char-set-difference` cs1 cs2 ... -> char-set
`char-set-xor` cs1 ... -> char-set
`char-set-diff+intersection` cs1 cs2 ... -> [char-set char-set]
  These procedures implement set complement, union, intersection, difference, and exclusive-or for character sets. The union, intersection and xor operations are n-ary. The difference function is also n-ary, associates to the left (that is, it computes the difference between its first argument and the union of all the other arguments), and requires at least one argument.

  Boundary cases:

      (char-set-union) => char-set:empty
      (char-set-intersection) => char-set:full
      (char-set-xor) => char-set:empty
      (char-set-difference cs) => cs

  `char-set-diff+intersection` returns both the difference and the intersection of the arguments -- it partitions its first parameter. It is equivalent to

      (values (char-set-difference cs1 cs2 ...)
              (char-set-intersection cs1 (char-set-union cs2 ...)))

  but can be implemented more efficiently.

  Programmers should be aware that `char-set-complement` could potentially be a very expensive operation in Scheme implementations that provide a very large character type, such as 32-bit Unicode. If this is a possibility, sets can be complimented with respect to a smaller universe using `char-set-difference`.

`char-set-complement!` cs -> char-set
`char-set-union!` cs1 cs2 ... -> char-set
`char-set-intersection!` cs1 cs2 ... -> char-set
`char-set-difference!` cs1 cs2 ... -> char-set
`char-set-xor!` cs1 cs2 ... -> char-set
`char-set-diff+intersection!` cs1 cs2 cs3 ... -> [char-set char-set]
  These are linear-update variants of the set-algebra functions. They are allowed, but not required, to side-effect their first (required) parameter.

  `char-set-diff+intersection!` is allowed to side-effect both of its two required parameters, cs1 and cs2.

## Standard character sets

Several character sets are predefined for convenience:

`char-set:lower-case` | Lower-case letters
`char-set:upper-case` | Upper-case letters
`char-set:title-case` | Title-case letters
`char-set:letter` | Letters
`char-set:digit` | Digits
`char-set:letter+digit` | Letters and digits
`char-set:graphic` | Printing characters except spaces
`char-set:printing` | Printing characters including spaces
`char-set:whitespace` | Whitespace characters
`char-set:iso-control` | The ISO control characters
`char-set:punctuation` | Punctuation characters
`char-set:symbol` | Symbol characters
`char-set:hex-digit` | A hexadecimal digit: 0-9, A-F, a-f
`char-set:blank` | Blank characters -- horizontal whitespace
`char-set:ascii` | All characters in the ASCII set.
`char-set:empty` | Empty set
`char-set:full` | All characters

Note that there may be characters in `char-set:letter` that are neither upper or lower case---this might occur in implementations that use a character type richer than ASCII, such as Unicode. A "graphic character" is one that would put ink on your page. While the exact composition of these sets may vary depending upon the character type provided by the underlying Scheme system, here are the definitions for some of the sets in an ASCII implementation:

`char-set:lower-case` | a-z
`char-set:upper-case` | A-Z
`char-set:letter` | A-Z and a-z
`char-set:digit` | 0123456789
`char-set:punctuation` | `!"#%&'()*,-./:;?@[\]_{}`
`char-set:symbol` | `$+<=>^`|~`
`char-set:whitespace` | Space, newline, tab, form feed,
| vertical tab, carriage return
`char-set:blank` | Space and tab
`char-set:graphic` | letter + digit + punctuation + symbol
`char-set:printing` | graphic + whitespace
`char-set:iso-control` | ASCII 0-31 and 127

Note that the existence of the `char-set:ascii` set implies that the underlying character set is required to be at least as rich as ASCII (including ASCII's control characters).

Rationale: The name choices reflect a shift from the older "alphabetic/numeric" terms found in R5RS and Posix to newer, Unicode-influenced "letter/digit" lexemes.

# Unicode, Latin-1 and ASCII definitions of the standard character sets

In Unicode Scheme implementations, the base character sets are compatible with Java's Unicode specifications. For ASCII or Latin-1, we simply restrict the Unicode set specifications to their first 128 or 256 codes, respectively. Scheme implementations that are not based on ASCII, Latin-1 or Unicode should attempt to preserve the sense or spirit of these definitions.

The following descriptions frequently make reference to the "Unicode character database." This is a file, available at URL

ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

Each line contains a description of a Unicode character. The first semicolon-delimited field of the line gives the hex value of the character's code; the second field gives the name of the character, and the third field gives a two-letter category. Other fields give simple 1-1 case-mappings for the character and other information; see

ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.html

for further description of the file's format. Note in particular the two-letter category specified in the third field, which is referenced frequently in the descriptions below.

## char-set:lower-case

For Unicode, we follow Java's specification: a character is lowercase if

- it is not in the range [U+2000,U+2FFF], and
- the Unicode attribute table does not give a lowercase mapping for it, and
- at least one of the following is true:
  - the Unicode attribute table gives a mapping to uppercase for the character, or
  - the name for the character in the Unicode attribute table contains the words "SMALL LETTER" or "SMALL LIGATURE".

The lower-case ASCII characters are

abcdefghijklmnopqrstuvwxyz

Latin-1 adds another 33 lower-case characters to the ASCII set:

00B5 | MICRO SIGN
00DF | LATIN SMALL LETTER SHARP S
00E0 | LATIN SMALL LETTER A WITH GRAVE
00E1 | LATIN SMALL LETTER A WITH ACUTE
00E2 | LATIN SMALL LETTER A WITH CIRCUMFLEX
00E3 | LATIN SMALL LETTER A WITH TILDE
00E4 | LATIN SMALL LETTER A WITH DIAERESIS
00E5 | LATIN SMALL LETTER A WITH RING ABOVE
00E6 | LATIN SMALL LETTER AE
00E7 | LATIN SMALL LETTER C WITH CEDILLA
00E8 | LATIN SMALL LETTER E WITH GRAVE
00E9 | LATIN SMALL LETTER E WITH ACUTE
00EA | LATIN SMALL LETTER E WITH CIRCUMFLEX
00EB | LATIN SMALL LETTER E WITH DIAERESIS
00EC | LATIN SMALL LETTER I WITH GRAVE
00ED | LATIN SMALL LETTER I WITH ACUTE
00EE | LATIN SMALL LETTER I WITH CIRCUMFLEX
00EF | LATIN SMALL LETTER I WITH DIAERESIS
00F0 | LATIN SMALL LETTER ETH
00F1 | LATIN SMALL LETTER N WITH TILDE
00F2 | LATIN SMALL LETTER O WITH GRAVE
00F3 | LATIN SMALL LETTER O WITH ACUTE
00F4 | LATIN SMALL LETTER O WITH CIRCUMFLEX
00F5 | LATIN SMALL LETTER O WITH TILDE
00F6 | LATIN SMALL LETTER O WITH DIAERESIS
00F8 | LATIN SMALL LETTER O WITH STROKE
00F9 | LATIN SMALL LETTER U WITH GRAVE
00FA | LATIN SMALL LETTER U WITH ACUTE
00FB | LATIN SMALL LETTER U WITH CIRCUMFLEX
00FC | LATIN SMALL LETTER U WITH DIAERESIS
00FD | LATIN SMALL LETTER Y WITH ACUTE
00FE | LATIN SMALL LETTER THORN
00FF | LATIN SMALL LETTER Y WITH DIAERESIS

Note that three of these have no corresponding Latin-1 upper-case character:

00B5 | MICRO SIGN
00DF | LATIN SMALL LETTER SHARP S
00FF | LATIN SMALL LETTER Y WITH DIAERESIS

(The compatibility micro character uppercases to the non-Latin-1 Greek capital mu; the German sharp s character uppercases to the pair of characters "SS," and the capital y-with-diaeresis is non-Latin-1.)

(Note that the Java spec for lowercase characters given at

http://java.sun.com/docs/books/jls/html/javalang.doc4.html#14345

is inconsistent. U+00B5 MICRO SIGN fulfills the requirements for a lower-case character (as of Unicode 3.0), but is not given in the numeric list of lower-case character codes.)

(Note that the Java spec for `isLowerCase()` given at

http://java.sun.com/products/jdk/1.2/docs/api/java/lang/Character.html#isLowerCase(char)

gives three mutually inconsistent definitions of "lower case." The first is the definition used in this SRFI. Following text says "A character is considered to be lowercase if and only if it is specified to be lowercase by the Unicode 2.0 standard (category Ll in the Unicode specification data file)." The former spec excludes U+00AA FEMININE ORDINAL INDICATOR and U+00BA MASCULINE ORDINAL INDICATOR; the later spec includes them. Finally, the spec enumerates a list of characters in the Latin-1 subset; this list excludes U+00B5 MICRO SIGN, which is included in both of the previous specs.)

## char-set:upper-case

For Unicode, we follow Java's specification: a character is uppercase if

- it is not in the range [U+2000,U+2FFF], and
- the Unicode attribute table does not give an uppercase mapping for it (this excludes titlecase characters), and
- at least one of the following is true:
  - the Unicode attribute table gives a mapping to lowercase for the character, or
  - the name for the character in the Unicode attribute table contains the words "CAPITAL LETTER" or "CAPITAL LIGATURE".

The upper-case ASCII characters are

ABCDEFGHIJKLMNOPQRSTUVWXYZ

Latin-1 adds another 30 upper-case characters to the ASCII set:

00C0 | LATIN CAPITAL LETTER A WITH GRAVE
00C1 | LATIN CAPITAL LETTER A WITH ACUTE
00C2 | LATIN CAPITAL LETTER A WITH CIRCUMFLEX
00C3 | LATIN CAPITAL LETTER A WITH TILDE
00C4 | LATIN CAPITAL LETTER A WITH DIAERESIS
00C5 | LATIN CAPITAL LETTER A WITH RING ABOVE
00C6 | LATIN CAPITAL LETTER AE
00C7 | LATIN CAPITAL LETTER C WITH CEDILLA
00C8 | LATIN CAPITAL LETTER E WITH GRAVE
00C9 | LATIN CAPITAL LETTER E WITH ACUTE
00CA | LATIN CAPITAL LETTER E WITH CIRCUMFLEX
00CB | LATIN CAPITAL LETTER E WITH DIAERESIS
00CC | LATIN CAPITAL LETTER I WITH GRAVE
00CD | LATIN CAPITAL LETTER I WITH ACUTE
00CE | LATIN CAPITAL LETTER I WITH CIRCUMFLEX
00CF | LATIN CAPITAL LETTER I WITH DIAERESIS
00D0 | LATIN CAPITAL LETTER ETH
00D1 | LATIN CAPITAL LETTER N WITH TILDE
00D2 | LATIN CAPITAL LETTER O WITH GRAVE
00D3 | LATIN CAPITAL LETTER O WITH ACUTE
00D4 | LATIN CAPITAL LETTER O WITH CIRCUMFLEX
00D5 | LATIN CAPITAL LETTER O WITH TILDE
00D6 | LATIN CAPITAL LETTER O WITH DIAERESIS
00D8 | LATIN CAPITAL LETTER O WITH STROKE
00D9 | LATIN CAPITAL LETTER U WITH GRAVE
00DA | LATIN CAPITAL LETTER U WITH ACUTE
00DB | LATIN CAPITAL LETTER U WITH CIRCUMFLEX
00DC | LATIN CAPITAL LETTER U WITH DIAERESIS
00DD | LATIN CAPITAL LETTER Y WITH ACUTE
00DE | LATIN CAPITAL LETTER THORN

## char-set:title-case

In Unicode, a character is titlecase if it has the category Lt in the character attribute database. There are very few of these characters; here is the entire 31-character list as of Unicode 3.0:

01C5 | LATIN CAPITAL LETTER D WITH SMALL LETTER Z WITH CARON
01C8 | LATIN CAPITAL LETTER L WITH SMALL LETTER J
01CB | LATIN CAPITAL LETTER N WITH SMALL LETTER J
01F2 | LATIN CAPITAL LETTER D WITH SMALL LETTER Z
1F88 | GREEK CAPITAL LETTER ALPHA WITH PSILI AND PROSGEGRAMMENI
1F89 | GREEK CAPITAL LETTER ALPHA WITH DASIA AND PROSGEGRAMMENI
1F8A | GREEK CAPITAL LETTER ALPHA WITH PSILI AND VARIA AND PROSGEGRAMMENI
1F8B | GREEK CAPITAL LETTER ALPHA WITH DASIA AND VARIA AND PROSGEGRAMMENI
1F8C | GREEK CAPITAL LETTER ALPHA WITH PSILI AND OXIA AND PROSGEGRAMMENI
1F8D | GREEK CAPITAL LETTER ALPHA WITH DASIA AND OXIA AND PROSGEGRAMMENI
1F8E | GREEK CAPITAL LETTER ALPHA WITH PSILI AND PERISPOMENI AND PROSGEGRAMMENI
1F8F | GREEK CAPITAL LETTER ALPHA WITH DASIA AND PERISPOMENI AND PROSGEGRAMMENI
1F98 | GREEK CAPITAL LETTER ETA WITH PSILI AND PROSGEGRAMMENI
1F99 | GREEK CAPITAL LETTER ETA WITH DASIA AND PROSGEGRAMMENI
1F9A | GREEK CAPITAL LETTER ETA WITH PSILI AND VARIA AND PROSGEGRAMMENI
1F9B | GREEK CAPITAL LETTER ETA WITH DASIA AND VARIA AND PROSGEGRAMMENI
1F9C | GREEK CAPITAL LETTER ETA WITH PSILI AND OXIA AND PROSGEGRAMMENI
1F9D | GREEK CAPITAL LETTER ETA WITH DASIA AND OXIA AND PROSGEGRAMMENI
1F9E | GREEK CAPITAL LETTER ETA WITH PSILI AND PERISPOMENI AND PROSGEGRAMMENI
1F9F | GREEK CAPITAL LETTER ETA WITH DASIA AND PERISPOMENI AND PROSGEGRAMMENI
1FA8 | GREEK CAPITAL LETTER OMEGA WITH PSILI AND PROSGEGRAMMENI
1FA9 | GREEK CAPITAL LETTER OMEGA WITH DASIA AND PROSGEGRAMMENI
1FAA | GREEK CAPITAL LETTER OMEGA WITH PSILI AND VARIA AND PROSGEGRAMMENI
1FAB | GREEK CAPITAL LETTER OMEGA WITH DASIA AND VARIA AND PROSGEGRAMMENI
1FAC | GREEK CAPITAL LETTER OMEGA WITH PSILI AND OXIA AND PROSGEGRAMMENI
1FAD | GREEK CAPITAL LETTER OMEGA WITH DASIA AND OXIA AND PROSGEGRAMMENI
1FAE | GREEK CAPITAL LETTER OMEGA WITH PSILI AND PERISPOMENI AND PROSGEGRAMMENI
1FAF | GREEK CAPITAL LETTER OMEGA WITH DASIA AND PERISPOMENI AND PROSGEGRAMMENI
1FBC | GREEK CAPITAL LETTER ALPHA WITH PROSGEGRAMMENI
1FCC | GREEK CAPITAL LETTER ETA WITH PROSGEGRAMMENI
1FFC | GREEK CAPITAL LETTER OMEGA WITH PROSGEGRAMMENI

There are no ASCII or Latin-1 titlecase characters.

## char-set:letter

In Unicode, a letter is any character with one of the letter categories (Lu, Ll, Lt, Lm, Lo) in the Unicode character database.

There are 52 ASCII letters

abcdefghijklmnopqrstuvwxyz
ABCDEFGHIJKLMNOPQRSTUVWXYZ

There are 117 Latin-1 letters. These are the 115 characters that are members of the Latin-1 `char-set:lower-case` and `char-set:upper-case` sets, plus

00AA | FEMININE ORDINAL INDICATOR
00BA | MASCULINE ORDINAL INDICATOR

(These two letters are considered lower-case by Unicode, but not by Java or SRFI 14.)

## char-set:digit

In Unicode, a character is a digit if it has the category Nd in the character attribute database. In Latin-1 and ASCII, the only such characters are 0123456789. In Unicode, there are other digit characters in other code blocks, such as Gujarati digits and Tibetan digits.

## char-set:hex-digit

The only hex digits are 0123456789abcdefABCDEF.

## char-set:letter+digit

The union of `char-set:letter` and `char-set:digit.`

## char-set:graphic

A graphic character is one that would put ink on paper. The ASCII and Latin-1 graphic characters are the members of

`char-set:letter`
`char-set:digit`
`char-set:punctuation`
`char-set:symbol`

## char-set:printing

A printing character is one that would occupy space when printed, i.e., a graphic character or a space character. `char-set:printing` is the union of `char-set:whitespace` and `char-set:graphic.`

## char-set:whitespace

In Unicode, a whitespace character is either

- a character with one of the space, line, or paragraph separator categories (Zs, Zl or Zp) of the Unicode character database.
- U+0009 Horizontal tabulation (\t control-I)
- U+000A Line feed (\n control-J)
- U+000B Vertical tabulation (\v control-K)
- U+000C Form feed (\f control-L)
- U+000D Carriage return (\r control-M)

There are 24 whitespace characters in Unicode 3.0:

0009 | HORIZONTAL TABULATION | \t control-I
000A | LINE FEED | \n control-J
000B | VERTICAL TABULATION | \v control-K
000C | FORM FEED | \f control-L
000D | CARRIAGE RETURN | \r control-M
0020 | SPACE | Zs
00A0 | NO-BREAK SPACE | Zs
1680 | OGHAM SPACE MARK | Zs
2000 | EN QUAD | Zs
2001 | EM QUAD | Zs
2002 | EN SPACE | Zs
2003 | EM SPACE | Zs
2004 | THREE-PER-EM SPACE | Zs
2005 | FOUR-PER-EM SPACE | Zs
2006 | SIX-PER-EM SPACE | Zs
2007 | FIGURE SPACE | Zs
2008 | PUNCTUATION SPACE | Zs
2009 | THIN SPACE | Zs
200A | HAIR SPACE | Zs
200B | ZERO WIDTH SPACE | Zs
2028 | LINE SEPARATOR | Zl
2029 | PARAGRAPH SEPARATOR | Zp
202F | NARROW NO-BREAK SPACE | Zs
3000 | IDEOGRAPHIC SPACE | Zs

The ASCII whitespace characters are the first six characters in the above list -- line feed, horizontal tabulation, vertical tabulation, form feed, carriage return, and space. These are also exactly the characters recognised by the Posix `isspace()` procedure. Latin-1 adds the no-break space.

Note: Java's `isWhitespace()` method is incompatible, including

0009 | HORIZONTAL TABULATION | (\t control-I)
001C | FILE SEPARATOR | (control-\)
001D | GROUP SEPARATOR | (control-])
001E | RECORD SEPARATOR | (control-^)
001F | UNIT SEPARATOR | (control-_)

and excluding

00A0 | NO-BREAK SPACE

Java's excluding the no-break space means that tokenizers can simply break character streams at "whitespace" boundaries. However, the exclusion introduces exceptions in other places, e.g. `char-set:printing` is no longer simply the union of `char-set:graphic` and `char-set:whitespace.`

## char-set:iso-control

The ISO control characters are the Unicode/Latin-1 characters in the ranges [U+0000,U+001F] and [U+007F,U+009F].

ASCII restricts this set to the characters in the range [U+0000,U+001F] plus the character U+007F.

Note that Unicode defines other control characters which do not belong to this set (hence the qualifying prefix "iso-" in the name). This restriction is compatible with the Java `IsISOControl()` method.

## char-set:punctuation

In Unicode, a punctuation character is any character that has one of the punctuation categories in the Unicode character database (Pc, Pd, Ps, Pe, Pi, Pf, or Po.)

ASCII has 23 punctuation characters:

    !"#%&'()*,-./:;?@[\]_{}

Latin-1 adds six more:

00A1 | INVERTED EXCLAMATION MARK
00AB | LEFT-POINTING DOUBLE ANGLE QUOTATION MARK
00AD | SOFT HYPHEN
00B7 | MIDDLE DOT
00BB | RIGHT-POINTING DOUBLE ANGLE QUOTATION MARK
00BF | INVERTED QUESTION MARK

Note that the nine ASCII characters `$+<=>^`|~` are not punctuation. They are "symbols."

## char-set:symbol

In Unicode, a symbol is any character that has one of the symbol categories in the Unicode character database (Sm, Sc, Sk, or So). There are nine ASCII symbol characters:

    $+<=>^`|~

Latin-1 adds 18 more:

00A2 | CENT SIGN
00A3 | POUND SIGN
00A4 | CURRENCY SIGN
00A5 | YEN SIGN
00A6 | BROKEN BAR
00A7 | SECTION SIGN
00A8 | DIAERESIS
00A9 | COPYRIGHT SIGN
00AC | NOT SIGN
00AE | REGISTERED SIGN
00AF | MACRON
00B0 | DEGREE SIGN
00B1 | PLUS-MINUS SIGN
00B4 | ACUTE ACCENT
00B6 | PILCROW SIGN
00B8 | CEDILLA
00D7 | MULTIPLICATION SIGN
00F7 | DIVISION SIGN

## char-set:blank

Blank chars are horizontal whitespace. In Unicode, a blank character is either

- a character with the space separator category (Zs) in the Unicode character database.
- U+0009 Horizontal tabulation (\t control-I)

There are eighteen blank characters in Unicode 3.0:

0009 | HORIZONTAL TABULATION | \t control-I
0020 | SPACE | Zs
00A0 | NO-BREAK SPACE | Zs
1680 | OGHAM SPACE MARK | Zs
2000 | EN QUAD | Zs
2001 | EM QUAD | Zs
2002 | EN SPACE | Zs
2003 | EM SPACE | Zs
2004 | THREE-PER-EM SPACE | Zs
2005 | FOUR-PER-EM SPACE | Zs
2006 | SIX-PER-EM SPACE | Zs
2007 | FIGURE SPACE | Zs
2008 | PUNCTUATION SPACE | Zs
2009 | THIN SPACE | Zs
200A | HAIR SPACE | Zs
200B | ZERO WIDTH SPACE | Zs
202F | NARROW NO-BREAK SPACE | Zs
3000 | IDEOGRAPHIC SPACE | Zs

The ASCII blank characters are the first two characters above -- horizontal tab and space. Latin-1 adds the no-break space.

Java doesn't have the concept of "blank" characters, so there are no compatibility issues.

# Reference implementation

This SRFI comes with a reference implementation. It resides at:

http://srfi.schemers.org/srfi-14/srfi-14.scm

I have placed this source on the Net with an unencumbered, "open" copyright. Some of the code in the reference implementation bears a distant family relation to the MIT Scheme implementation, and being derived from that code, is covered by the MIT Scheme copyright (which is a generic BSD-style open-source copyright -- see the source file for details). The remainder of the code was written by myself for scsh or for this SRFI; I have placed this code under the scsh copyright, which is also a generic BSD-style open-source copyright.

The code is written for portability and should be simple to port to any Scheme. It has only the following deviations from R4RS, clearly discussed in the comments:

- an `error` procedure;
- the R5RS `values` procedure for producing multiple return values;
- a simple `check-arg` procedure for argument checking;
- `let-optionals*` and `:optional` macros for for parsing, checking and defaulting optional arguments from rest lists;
- The SRFI-19 `define-record-type` form;
- `bitwise-and` for the hash function;
- `%latin1->char` and `%char->latin1`.

The library is written for clarity and well-commented; the current source is about 375 lines of source code and 375 lines of comments and white space. It is also written for efficiency. Fast paths are provided for common cases.

This is not to say that the implementation can't be tuned up for a specific Scheme implementation. There are notes in comments addressing ways implementors can tune the reference implementation for performance.

In short, I've written the reference implementation to make it as painless as possible for an implementor -- or a regular programmer -- to adopt this library and get good results with it.

The code uses a rather simple-minded, inefficient representation for ASCII/Latin-1 char-sets -- a 256-character string. The character whose code is i is in the set if s[i] = ASCII 1 (soh, or ^a); not in the set if s[i] = ASCII 0 (nul). A much faster and denser representation would be 16 or 32 bytes worth of bit string. A portable implementation using bit sets awaits standards for bitwise logical-ops and byte vectors.

"Large" character types, such as Unicode, should use a sparse representation, taking care that the Latin-1 subset continues to be represented with a dense 32-byte bit set.

# Acknowledgements

The design of this library benefited greatly from the feedback provided during the SRFI discussion phase. Among those contributing thoughtful commentary and suggestions, both on the mailing list and by private discussion, were Paolo Amoroso, Lars Arvestad, Alan Bawden, Jim Bender, Dan Bornstein, Per Bothner, Will Clinger, Brian Denheyer, Kent Dybvig, Sergei Egorov, Marc Feeley, Matthias Felleisen, Will Fitzgerald, Matthew Flatt, Arthur A. Gleckler, Ben Goetter, Sven Hartrumpf, Erik Hilsdale, Shiro Kawai, Richard Kelsey, Oleg Kiselyov, Bengt Kleberg, Donovan Kolbly, Bruce Korb, Shriram Krishnamurthi, Bruce Lewis, Tom Lord, Brad Lucier, Dave Mason, David Rush, Klaus Schilling, Jonathan Sobel, Mike Sperber, Mikael Staldal, Vladimir Tsyshevsky, Donald Welsh, and Mike Wilson. I am grateful to them for their assistance.

I am also grateful the authors, implementors and documentors of all the systems mentioned in the introduction. Aubrey Jaffer should be noted for his work in producing Web-accessible versions of the R5RS spec, which was a tremendous aid.

This is not to imply that these individuals necessarily endorse the final results, of course.

During this document's long development period, great patience was exhibited by Mike Sperber, who is the editor for the SRFI, and by Hillary Sullivan, who is not.

# References & links

[Java]
  The following URLs provide documentation on relevant Java classes.
  http://java.sun.com/products/jdk/1.2/docs/api/java/lang/Character.html
  http://java.sun.com/products/jdk/1.2/docs/api/java/lang/String.html
  http://java.sun.com/products/jdk/1.2/docs/api/java/lang/StringBuffer.html
  http://java.sun.com/products/jdk/1.2/docs/api/java/text/Collator.html
  http://java.sun.com/products/jdk/1.2/docs/api/java/text/package-summary.html
[MIT-Scheme]
  http://www.swiss.ai.mit.edu/projects/scheme/
[R5RS]
  Revised5 report on the algorithmic language Scheme.
  R. Kelsey, W. Clinger, J. Rees (editors).
  Higher-Order and Symbolic Computation, Vol. 11, No. 1, September, 1998.
  and ACM SIGPLAN Notices, Vol. 33, No. 9, October, 1998.
  Available at http://www.schemers.org/Documents/Standards/.
[SRFI]
  The SRFI web site.
  http://srfi.schemers.org/
[SRFI-14]
  SRFI-14: String libraries.
  http://srfi.schemers.org/srfi-14/
  This document, in HTML:
    http://srfi.schemers.org/srfi-14/srfi-14.html
  This document, in plain text format:
    http://srfi.schemers.org/srfi-14/srfi-14.txt
  Source code for the reference implementation:
    http://srfi.schemers.org/srfi-14/srfi-14.scm
  Scheme 48 module specification, with typings:
    http://srfi.schemers.org/srfi-14/srfi-14-s48-module.scm
  Regression-test suite:
    http://srfi.schemers.org/srfi-14/srfi-14-tests.scm
[Unicode]
  http://www.unicode.org/
[UnicodeData]
  The Unicode character database.
  ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt
  ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.html

# Copyright

Certain portions of this document -- the specific, marked segments of text describing the R5RS procedures -- were adapted with permission from the R5RS report.

All other text is copyright (C) Olin Shivers (1998, 1999, 2000). All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.