ByteArray represents bytes, internally using one byte per value which make them space efficient. Because the data are binary, the performance gain of using them in place of string of bytes, is significant. Indeed, not all byte sequences are a valid String, so, when one stores bytes in a string, the data needs to be validated. That not required with ByteArray.

There is an effort to implement most if not all the features you've listed as top level function. In the mean time, here are some non documented functions that you may find interesting, as a complement to those you already mentioned:

• Developer`EncodeBase64: ByteArray to String convertion, takes a byte array, returns a base64 string.

In[1]:= Developer`EncodeBase64[ByteArray[Range[5]]]
Out[1]= "AQIDBAU="

• BinaryWrite accepts ByteArray, as an undocumented feature. I'm not aware of a reverse function that reads a file directly into a byte array.

Generally speaking, ByteArray is recommended everywhere one uses binary data. It leads me to talk about BinarySerialize. BinarySerialize serializes any Wolfram Language expression to a binary representation that is platform independent and fast to deserialize. Contrary to MX which contains all the definitions of a given expression, like DownValues, BinarySerialize is more data oriented, and only somehow represents the FullForm of an expression. The format used by BinarySerialize has simple enough specifications that we may consider publishing them.

As users noticed on StackOverflow, BinarySerialize does not always produce a smaller output with respect to ByteCount. One reason to that is, contrary to Compress, BinarySerialize does not automatically performs compression of the output. You pay the cost of a zlib compression only if needs be. Also some expressions like arrays (packed, raw) are already efficiently stored in memory, so having an output size of roughly the size of the byte count is generally a good result (Range produces packed array).

Do you mean the reverse, i.e. that not all Strings are a valid byte sequence? This gives True:

Let me rephrase what I wrote, I should have been clearer. You can indeed represent any unicode character codepoint from 0 to 65535 in a String. Internally though, characters are encoded on bytes, which requires to define a character encoding (i.e. a consistent way of representing values from 0 to 65535 using bytes). Given a character encoding, some byte sequences may be invalid.

e.g: In UTF-8 192 is not a valid byte and FromCharacterCode[192, "UTF-8"] returns an error.

From this follows that when you build a String out of bytes, its content is encoded. That's not required with byte arrays. I hope it clarifies.

It is possible to get the contents of a HDF5 file as a ByteArray, it is one of the import elements. But I do not know what sorts of conversions it goes through to get there.

As Dorian indicated, many of the requested features are in the works. Indeed, some of them were planned for the initialze release of ByteArray, but were delayed for various reasons. We certainly hope to gradually added them over the course the next few 11.x release.

A bit more of context and answers to questions (both excplicit and implicit).

1) ByteArrays are not really arrays (at least in the sense of the Wolfram Language). The represent one dimensional data of 8-bit unsigned integers. In that sense, ByteList or ByteRow might have been clearer. You can think of them as the closest thing WL has to a char *, from that POV of the name ByteArray makes sense.

2) I think it would be better to think of them as being like SparseArray or StructuredArray, as opposed to packed array. They are not transparent to user-level functions, but they overload many basic language constructs like Part, Take, etc so that that appear like a 1-dimensional list.

3) One very big difference between SparseArray/StructuredArray and ByteArray is that ByteArray is opaque to Listable functions. This is intentional, because we don't them to be accidentally converted something else. And even some thing as simple as Plus: what does addition mean? Do individual entries overflow? Does it get converted to a normal list? Does it get converter to some future TwoByteArray? (We certainly won't have something by that name, but the idea is clear enough.

4) Unlike PackedArray which has a FullForm like its normal list, ByteArray uses Base64 so that it efficiently pack its values when you Put / Get it to files, not just in memory.

5) We internally encode strings in a variant of UTF-8. Now, of course, any byte can be faithfully converted to/from ISO8859-1, but that encoding only equals UTF-8 for the lower 7 bits. For other values, you need to use multiple bytes per character. So using a string to store byte data is both less space efficient and time efficient (since you need to ensure to correct conversion between the two encodings.)

The kernel used to use UCS-2 internally. It now uses a variant of UTF-8. MathLink also gained functions for sending and transmitting / receiving UTF-8. These were steps 1 and 2 in the process for getting the full unicode character set. But there are several additional steps to actually get there. Important additional ones include creating equivalents of \:wxyz for non-BMP characters; getting the kernel, MathLink, and FE successfully talking to each other using these new methods; and finding all in the places where the assumption that characters lie in the range 0-65535 is hardcoded, either implictly or explicitly, and updating the code. We have made progress on some of these internally, but as you might imagine its an on-going process and we certainly can't promise the feature on any particular timeline.

Please do not forget about Unicode in file paths:

• How to workaround failures with Unicode filepaths?

You can't directly create a ByteArray from a RawArray. But, I see no reason not to support it, since we already have ByteArray from PackedArray.