Data.Text.ICU.Normalize

text-icu-0.7.0.1: Bindings to the ICU library

Copyright | (c) 2009, 2010 Bryan O'Sullivan
License | BSD-style
Maintainer | bos@serpentine.com
Stability | experimental
Portability | GHC
Safe Haskell | None
Language | Haskell98

Data.Text.ICU.Normalize

Contents

- Unicode normalization API
- Normalization functions
- Normalization checks
- Normalization-sensitive comparison

Description

Character set normalization functions for Unicode, implemented as bindings to the International Components for Unicode (ICU) libraries.

Synopsis

- data NormalizationMode
  - = None
  - | NFD
  - | NFKD
  - | NFC
  - | NFKC
  - | FCD
- normalize :: NormalizationMode -> Text -> Text
- quickCheck :: NormalizationMode -> Text -> Maybe Bool
- isNormalized :: NormalizationMode -> Text -> Bool
- data CompareOption
  - = InputIsFCD
  - | CompareIgnoreCase
  - | FoldCaseExcludeSpecialI
- compare :: [CompareOption] -> Text -> Text -> Ordering

# Unicode normalization API

The `normalize` function transforms Unicode text into an equivalent composed or decomposed form, allowing for easier sorting and searching of text. `normalize` supports the standard normalization forms described in http://www.unicode.org/unicode/reports/tr15/, Unicode Standard Annex #15: Unicode Normalization Forms.

Characters with accents or other adornments can be encoded in several different ways in Unicode. For example, take the character A-acute. In Unicode, this can be encoded as a single character (the "composed" form):

         00C1    LATIN CAPITAL LETTER A WITH ACUTE

or as two separate characters (the "decomposed" form):

         0041    LATIN CAPITAL LETTER A
         0301    COMBINING ACUTE ACCENT

To a user of your program, however, both of these sequences should be treated as the same "user-level" character "A with acute accent". When you are searching or comparing text, you must ensure that these two sequences are treated equivalently. In addition, you must handle characters with more than one accent. Sometimes the order of a character's combining accents is significant, while in other cases accent sequences in different orders are really equivalent.

Similarly, the string "ffi" can be encoded as three separate letters:

         0066    LATIN SMALL LETTER F
         0066    LATIN SMALL LETTER F
         0069    LATIN SMALL LETTER I

or as the single character

         FB03    LATIN SMALL LIGATURE FFI

The "ffi" ligature is not a distinct semantic character, and strictly speaking it shouldn't be in Unicode at all, but it was included for compatibility with existing character sets that already provided it. The Unicode standard identifies such characters by giving them "compatibility" decompositions into the corresponding semantic characters. When sorting and searching, you will often want to use these mappings.

`normalize` helps solve these problems by transforming text into the canonical composed and decomposed forms as shown in the first example above. In addition, you can have it perform compatibility decompositions so that you can treat compatibility characters the same as their equivalents. Finally, `normalize` rearranges accents into the proper canonical order, so that you do not have to worry about accent rearrangement on your own.

Form `FCD`, "Fast C or D", is also designed for collation. It allows to work on strings that are not necessarily normalized with an algorithm (like in collation) that works under "canonical closure", i.e., it treats precomposed characters and their decomposed equivalents the same.

It is not a normalization form because it does not provide for uniqueness of representation. Multiple strings may be canonically equivalent (their NFDs are identical) and may all conform to `FCD` without being identical themselves.

The form is defined such that the "raw decomposition", the recursive canonical decomposition of each character, results in a string that is canonically ordered. This means that precomposed characters are allowed for as long as their decompositions do not need canonical reordering.

Its advantage for a process like collation is that all `NFD` and most `NFC` texts - and many unnormalized texts - already conform to `FCD` and do not need to be normalized (`NFD`) for such a process. The `FCD` `quickCheck` will return `Yes` for most strings in practice.

``normalize` `FCD`` may be implemented with `NFD`.

For more details on `FCD` see the collation design document: http://source.icu-project.org/repos/icu/icuhtml/trunk/design/collation/ICU_collation_design.htm

ICU collation performs either `NFD` or `FCD` normalization automatically if normalization is turned on for the collator object. Beyond collation and string search, normalized strings may be useful for string equivalence comparisons, transliteration/transcription, unique representations, etc.

The W3C generally recommends to exchange texts in `NFC`. Note also that most legacy character encodings use only precomposed forms and often do not encode any combining marks by themselves. For conversion to such character encodings the Unicode text needs to be normalized to `NFC`. For more usage examples, see the Unicode Standard Annex.

data NormalizationMode #

Normalization modes.

Constructors

None | No decomposition/composition.
NFD | Canonical decomposition.
NFKD | Compatibility decomposition.
NFC | Canonical decomposition followed by canonical composition.
NFKC | Compatibility decomposition followed by canonical composition.
FCD | "Fast C or D" form.

Instances

Enum NormalizationMode # |
Methods succ :: NormalizationMode -> NormalizationMode # pred :: NormalizationMode -> NormalizationMode # toEnum :: Int -> NormalizationMode # fromEnum :: NormalizationMode -> Int # enumFrom :: NormalizationMode -> [NormalizationMode] # enumFromThen :: NormalizationMode -> NormalizationMode -> [NormalizationMode] # enumFromTo :: NormalizationMode -> NormalizationMode -> [NormalizationMode] # enumFromThenTo :: NormalizationMode -> NormalizationMode -> NormalizationMode -> [NormalizationMode] #
Eq NormalizationMode # |
Methods (==) :: NormalizationMode -> NormalizationMode -> Bool # (/=) :: NormalizationMode -> NormalizationMode -> Bool #
Show NormalizationMode # |
Methods showsPrec :: Int -> NormalizationMode -> ShowS # show :: NormalizationMode -> String # showList :: [NormalizationMode] -> ShowS #

# Normalization functions

normalize :: NormalizationMode -> Text -> Text #

Normalize a string according the specified normalization mode.

# Normalization checks

quickCheck :: NormalizationMode -> Text -> Maybe Bool #

Perform an efficient check on a string, to quickly determine if the string is in a particular normalization form.

A `Nothing` result indicates that a definite answer could not be determined quickly, and a more thorough check is required, e.g. with `isNormalized`. The user may have to convert the string to its normalized form and compare the results.

A result of `Just` `True` or `Just` `False` indicates that the string definitely is, or is not, in the given normalization form.

isNormalized :: NormalizationMode -> Text -> Bool #

Indicate whether a string is in a given normalization form.

Unlike `quickCheck`, this function returns a definitive result. For `NFD`, `NFKD`, and `FCD` normalization forms, both functions work in exactly the same ways. For `NFC` and `NFKC` forms, where `quickCheck` may return `Nothing`, this function will perform further tests to arrive at a definitive result.

# Normalization-sensitive comparison

data CompareOption #

Options to `compare`.

Constructors

InputIsFCD | The caller knows that both strings fulfill the `FCD` conditions. If not set, `compare` will `quickCheck` for `FCD` and normalize if necessary.
CompareIgnoreCase | Compare strings case-insensitively using case folding, instead of case-sensitively. If set, then the following case folding options are used.
FoldCaseExcludeSpecialI | When case folding, exclude the special I character. For use with Turkic (Turkish/Azerbaijani) text data.

Instances

Enum CompareOption # |
Methods succ :: CompareOption -> CompareOption # pred :: CompareOption -> CompareOption # toEnum :: Int -> CompareOption # fromEnum :: CompareOption -> Int # enumFrom :: CompareOption -> [CompareOption] # enumFromThen :: CompareOption -> CompareOption -> [CompareOption] # enumFromTo :: CompareOption -> CompareOption -> [CompareOption] # enumFromThenTo :: CompareOption -> CompareOption -> CompareOption -> [CompareOption] #
Eq CompareOption # |
Methods (==) :: CompareOption -> CompareOption -> Bool # (/=) :: CompareOption -> CompareOption -> Bool #
Show CompareOption # |
Methods showsPrec :: Int -> CompareOption -> ShowS # show :: CompareOption -> String # showList :: [CompareOption] -> ShowS #

compare :: [CompareOption] -> Text -> Text -> Ordering #

Compare two strings for canonical equivalence. Further options include case-insensitive comparison and code point order (as opposed to code unit order).

Canonical equivalence between two strings is defined as their normalized forms (`NFD` or `NFC`) being identical. This function compares strings incrementally instead of normalizing (and optionally case-folding) both strings entirely, improving performance significantly.

Bulk normalization is only necessary if the strings do not fulfill the `FCD` conditions. Only in this case, and only if the strings are relatively long, is memory allocated temporarily. For `FCD` strings and short non-`FCD` strings there is no memory allocation.