Module Uucp

module Uucp: sig .. end
Unicode character properties.

Uucp provides efficient access to a selection of character properties of the Unicode character database.

Consult the individual modules for sample code related to the properties. A minimal Unicode introduction is also available.

1.1.0-12-g15323f2 — Unicode version 8.0.0 — homepage


Unicode version

val unicode_version : string
unicode_version is the Unicode version supported by the library.


type uchar = int 
The type for Unicode characters. A value of this type must be an Unicode scalar value which is an integer in the ranges 0x00000xD7FF and 0xE0000x10FFFF. This can be asserted with Uucp.Uchar.is_uchar.
module Uchar: sig .. end


Consult information about the property distribution in modules and omissions.

Warning. The result of functions is undefined if their uchar arguments do not satisfy the Uucp.Uchar.is_uchar predicate.

module Age: sig .. end
Age property.
module Alpha: sig .. end
Alphabetic property.
module Block: sig .. end
Block property and block ranges.
module Break: sig .. end
Break properties.
module Case: sig .. end
Case properties, mappings and foldings.
module Cjk: sig .. end
CJK properties.
module Func: sig .. end
Function and graphics properties.
module Gc: sig .. end
General category property.
module Gen: sig .. end
General properties.
module Id: sig .. end
Identifier properties.
module Name: sig .. end
Name and name alias properties.
module Num: sig .. end
Numeric properties.
module Script: sig .. end
Script and script extensions properties.
module White: sig .. end
White space property.

Property module distribution and omissions

Properties are approximatively distributed in modules by scope of use like in this property index table. However some subset of properties live in their own modules.

Obsolete and deprecated properties are omitted. So are those related to normalization, shaping and bidirectionality. Here is the full list of omitted properties, if you think one of these property should be added get in touch with a rationale.

Minimal Unicode introduction

Characters — if they exist

The purpose of Unicode is to have a universal way of representing characters of writing systems known to the world in computer systems. Defining the notion of character is a very complicated question with both philosophical and political implications. To side step these issues, we only talk about characters from a programmer's point of view and simply say that the purpose of Unicode is to assign meaning to the integers of a well-defined integer range.

This range is called the Unicode codespace, it spans from 0x0000 to 0x10FFFF and its boundaries are cast in stone. Members of this range are called Unicode code points. Note that an OCaml int value can represent them on both 32- and 64-bit platforms.

There's a lot of (non-exclusive) terminology predicates that can be applied to code points. I will only mention the most useful ones here.

First there are the reserved or unassigned code points, those are the integers to which the standard doesn't assign any meaning yet. They are reserved for future assignment and may become meaningful in newer versions of the standard. Be aware that once a code point has been assigned (aka as encoded) by the standard most of its properties may never change again, see the stability policy for details.

A very important subset of code points are the Unicode scalar values, these are the code points that belong to the ranges 0x00000xD7FF and 0xE0000x10FFFF. This is the complete Unicode codespace minus the range 0xD8000xDFFF of so called surrogate code points, a hack to be able to encode all scalar values in UTF-16 (more on that below).

Scalar values are what I call, by a total abuse of terminology, the Unicode characters; it is what a proper uchar type should represent. From a programmer's point of view they are the sole integers you will have to deal with during processing and the only code points that you are allowed to serialize and deserialize to valid Unicode byte sequences.

Unicode uses a standard notation to denote code points in running text. A code point is expressed as U+n where n is four to six uppercase hexadecimal digits with leading zeros omitted unless the code point has fewer than four digits (in printf words "U+%04X"). For example the code point bounds are expressed by U+0000 and U+10FFFF and the surrogate bounds by U+D800 and U+DFFF.

Interlude — what is assigned ?

Lots of the world's scripts are encoded in the standard. The code charts give a precise idea of the coverage.

In order to be sucessful Unicode decided to be inclusive and to contain pre-existing international and national standards. For example the scalar values from U+0000 to U+007F correspond exactly to the code values of characters encoded by the US-ASCII standard, while those from U+0000 to U+00FF correspond exactly to the code values of ISO-8859-1 (latin1). Many other standard are injected into the codespace but their map to Unicode scalar values may not be as straightforward as the two examples given above.

One thing to be aware of is that because of the inclusive nature of the standard the same abstract character may be represented in more than one way by the standard. A simple example is the latin character "é", which can either be represented by the single scalar value U+00E9 or by the sequence of scalar values <U+0065, U+0301> that is a latin small letter "e" followed by the combining acute accent "´". This non uniqueness of representation is problematic, for example whenever you want to test sequences of scalar values for equality. Unicode solves this by defining equivalence classes between sequences of scalar values, this is called Unicode normalization and we will talk about it later.

Another issue is character spoofing. Many encoded characters ressemble each other when displayed but have different scalar values and meaning. The Unicode Security FAQ has more information and pointers about these issues.

Serializing integers — UTF-X

There is more than one way of representing a large integer as a sequence of bytes. The Unicode standard defines seven encoding schemes, also known as Unicode transformation formats (UTF), that precisely define how to encode and decode scalar values — take note, scalar values, not code points — as byte sequences.

The cost of using one representation over the other depends on the character usage. For example UTF-8 is fine for latin scripts but wasteful for east-asian scripts, while the converse is true for UTF-16. I never saw any usage of UTF-32 on disk or wires, it is very wasteful. However, in memory, UTF-32 has the advantage that characters become directly indexable.

For more information see the Unicode UTF-8, UTF-16, UTF-32 and BOM FAQ.

Interlude — Useful scalar values

The following scalar values are useful to know:

Equivalence and normalization

We mentioned above that concrete textual data may be represented by more than one sequence of scalar values. Latin letters with diacritics are a simple example of that. In order to be able to test two sequences of scalar values for equality we should be able to ignore these differences. The easiest way to do so is to convert them to a normal form where these differences are removed and then use binary equality to test them.

However first we need to define a notion of equality between sequences. Unicode defines two of them, which one to use depends on your processing context.

Canonical equivalence is included in compatiblity equivalence: two canonically equivalent sequences are also compatibility equivalent, but the converse may not be true.

A normal form is a function mapping a sequence of scalar values to a sequence of scalar values. The Unicode standard defines four different normal forms, the one to use depends on the equivalence you want and your processing context:

Once you have two sequences in a known normal form you can compare them using binary equality. If the normal form is NFD or NFC, binary equality will entail canonical equivalence of the sequences. If the normal form is NFKC or NFKD equality will entail compatibility equivalence of the sequences. Note that normal forms are not closed under concatenation: if you concatenate two sequence of scalar values you have to renormalize the result.

For more information about normalization, see the Normalization FAQ.

Collation — sorting in alphabetical order

Normalisation forms allow to define a total order between sequences of scalar values using binary comparison. However this order is purely arbitrary. It has no meaning because the magnitude of a scalar value has, in general, no meaning. The process of ordering sequences of scalar values in a standard order like alphabetical order is called collation. Unicode defines a customizable algorithm to order two sequences of scalar values in a meaningful way, the Unicode collation algorithm. For more information and further pointers see the Unicode Collation FAQ.

Biased tips for OCaml programs and libraries

Character data as UTF-8 encoded OCaml strings. For most OCaml programs it will be entirely sufficient to deal with Unicode by just treating the byte sequence of regular OCaml strings as valid UTF-8 encoded data.

Many libraries will already return you character data under this representation. Besides latin1 identifiers having been deprecated in OCaml 4.01, UTF-8 encoding your sources allows you to write UTF-8 encoded string literals directly in your programs. Be aware though that as far as OCaml's compiler is concerned these are just sequences of bytes and you can't trust these strings to be valid UTF-8 as they depend on how correctly your editor encodes them. That is unless you escape their valid UTF-8 bytes explicitely (e.g. "\xF0\x9F\x90\xAB" is the correct encoding of U+1F42B), you will need to validate them and most likely normalize them.

Checking the validity of UTF-8 strings should only be performed at the boundaries of your program: on your string literals, on data input or on the results of untrusted libraries (be careful, some libraries like Yojson will happily return you invalid UTF-8 strings). This allows you to only deal with valid UTF-8 throughout your program and avoid redundant validity checks, internally or on output. The following properties of UTF-8 are useful to remember:

For checking validity or recode the other UTF encoding schemes into UTF-8 encoded OCaml strings, the Uutf module can be used. It will also be useful if you need to fold over the scalar values of your UTF-8 encoded strings, or build new UTF-8 strings from scalar values.

UTF-8 and ASCII. As mentioned above, each of the 128 US-ASCII characters is represented by its own US-ASCII byte representation in UTF-8. So if you want to look for an US-ASCII character in an UTF-8 encoded string, you can just scan the bytes. But beware on the nature of your data and the algorithm you need to implement. For example to detect spaces in the string, looking for the US-ASCII space U+0020 may not be sufficient, there are a lot of other space characters like the no break space U+00A0 that are beyond the US-ASCII repertoire. Folding over the scalar values with Uutf and checking them with Uucp.White.is_white_space is a better idea. Same holds for line breaks, see for example Uutf.nln and Uutf.readlines for more information about these issues.

Equating and comparing UTF-8 encoded OCaml strings. If you understood well the above section about equivalence and normalization you should realise that blindly comparing UTF-8 encoded OCaml strings using won't bring you anywhere if you don't normalize them before. The Uunf module can be used for that. Don't forget that normalization is not closed under string concatenation.

Using on normalized UTF-8 encoded OCaml strings defines a total order on them that you can use with the Map or Set modules as long as you are not interested in the actual meaning of the order.

If you are looking for case insensitive equality have a look at the sample code of the Uucp.Case module.

Sort strings alphabetically. The only solution at the moment for collating strings is to use Camomile but be aware that it supports only Unicode 3.2 character data so don't be surprised if newer scripts don't order correctly. The official collation data also has been significantly tweaked since then.

Range processing. Forget about trying to process Unicode characters using hard coded ranges of scalar values like it was possible to do with US-ASCII. The Unicode standard is not closed, it is evolving, new characters are being assigned. This makes it impossible to derive properties based simply on their integer value or position in ranges of characters. That's the reason why we have the Unicode character database and Uucp to access their properties. Using Uucp.White.is_white_space will be future proof should a new character deemed white be added to the standard (both Uucp and your progam will need a recompile though).

Transcoding. Transcoding from legacy encodings to Unicode may be quite involved, use Camomile if you need to do that. There is however one translation that is very easy and direct: it is the one from ISO 8859-1 also known as latin1, the default encoding of OCaml chars. latin1 having been encoded in Unicode in the range of scalar values U+0000 to U+00FF which corresponds to latin1 code value, the translation is trivial, it is the identity:

let char_to_scalar_value c = Char.code c
let char_of_scalar_value s =
    if s > 255 then invalid_arg "" (* can't represent *) else
    Char.chr s

Pretty-printing code points in ASCII "U+%04X" is an OCaml formatting string for printing an US-ASCII representation of an Unicode code point according to the standards' notational conventions.

OCaml libraries. If you write a library that deals with textual data, you should, unless technically impossible, always interact with the client of the library using Unicode. If there are other encodings involved transcode them to/from Unicode so that the client needs only to deal with Unicode, the burden of dealing with the encoding mess has to be on the library, not the client.

In this case there is no absolute need to depend on an Unicode text data structure, just use valid UTF-8 encoded data as OCaml strings. Specify clearly in the documentation that all the strings returned by or given to the library must be valid UTF-8 encoded data. The validity contract is important for performance reasons, it allows the client to trust the string and avoid performing redundant checks and the library to trust the strings it was given without having to perform further checks. Remember that concatenating to UTF-8 valid strings results in an UTF-8 valid string.