Uucp provides efficient access to a selection of character
properties of the Unicode character database.
v2.0.0 — Unicode version 9.0.0 — homepage
Consult information about the property distribution
in modules and omissions.
val unicode_version :
unicode_versionis the Unicode version supported by the library.
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
0xDFFF 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. Since OCaml 4.03 the
standard library defines an
Uchar.t type to represent them.
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.
Since OCaml 4.03 the standard library defines the
which represents Unicode scalar values. Support for
previous OCaml versions is provided by the
uchar OPAM/ocamlfind compatibility
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 Unicode text 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 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 via
As mentioned in Serializing integers — UTF-X, 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
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 issues.
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
Uunf module can be used for
that. Remember that concatenating normalized strings does not
result in a normalized string.
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.
For case insensitive equality have a look at the
sample code of the
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.
Uuseg module implements the
Unicode text segmentation
algorithms to find user-perceived character, word and sentence
boundaries in Unicode text. It also provides an implementation of
the Unicode Line Breaking
Algorithm to find line breaks and line break opportunities.
Among other things the
Uuseg_string module uses these
algorithms to provide OCaml standard library formatters
for best-effort formatting of UTF-8 encoded strings.
readline function as mandated by the Unicode standard is available
Uutf's sample code.
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
your progam will need a recompile though).
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
"U+%04X" is an OCaml formatting string for printing an US-ASCII
representation of an Unicode code point according to the
standards' notational conventions. This is what the standard library
Uchar.dump formatter does for
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
strings and the standard library
Specify clearly in the documentation that all the
returned by or given to the library must be valid UTF-8 encoded
data. This validity contract is important for performance reasons:
it allows both the client and the library to trust the string and
forgo redundant validity checks. Remember that concatenating valid
UTF-8 strings results in valid UTF-8 string.