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
val unicode_version :
unicode_versionis the Unicode version supported by the library.
0x10FFFF. This can be asserted with
Consult information about the property distribution in modules and omissions.
Warning. The result of functions is undefined if their
arguments do not satisfy the
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.
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
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
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
0x10FFFF. This is the
complete Unicode codespace minus the range
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
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
"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.
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.
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.
(0xFF,0xFE), indicating UTF-16LE, or
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.
The following scalar values are useful to know:
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.
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.
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:
Uutfmodule 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
Uutf and checking them with
Uucp.White.is_white_space is a
better idea. Same holds for line breaks, see for example
Uutf.readlines for more information about these
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
Pervasives.compare won't bring you
anywhere if you don't normalize them before. The
can be used for that. Don't forget that normalization is not
closed under string concatenation.
Pervasives.compare on normalized UTF-8 encoded OCaml
strings defines a total order on them that you can use with the
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
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
Uucp.White.is_white_space will be future proof should
a new character deemed white be added to the standard (both
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
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
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.