There is a very good reason I use 8-bit characters in Aspell. Speed and simplicity. While many parts of my code can fairly easily be converted to some sort of wide character as my code is clean. Other parts cannot be.
One of the reasons why is because in many, many places I use a direct lookup to find out various information about characters. With 8-bit characters this is very feasible because there is only 256 of them. With 16-bit wide characters this will waste a LOT of space. With 32-bit characters this is just plain impossible. Converting the lookup tables to another form is certainly possible but degrades performance significantly.
Furthermore, some of my algorithms rely on words consisting only on a small number of distinct characters (often around 30 when case and accents are not considered). When the possible character can consist of any Unicode character this number becomes several thousand, if that. In order for these algorithms to still be used, some sort of limit will need to be placed on the possible characters the word can contain. If I impose that limit, I might as well use some sort of 8-bit characters set which will automatically place the limit on what the characters can be.
There is also the issue of how I should store the word lists in memory? As a string of 32 bit wide characters. Now that is using up 4 times more memory than characters would and for languages that can fit within an 8-bit character that is, in my view, a gross waste of memory. So maybe I should store them is some variable width format such as UTF-8. Unfortunately, way, way too many of the algorithms will simply not work with variable width characters without significant modification which will very likely degrade performance. So the solution is to work with the characters as 32-bit wide characters and then convert it to a shorter representation when storing them in the lookup tables. Now that can lead to an inefficiency. I could also use 16 bit wide characters, however that may not be good enough to hold all future versions of Unicode and therefore has the same problems.
As a response to the space waste used by storing word lists in some sort of wide format some one asked:
Since hard drives are cheaper and cheaper, you could store a dictionary in a usable (uncompressed) form and use it directly with memory mapping. Then the efficiency would directly depend on the disk caching method, and only the used part of the dictionaries would really be loaded into memory. You would no more have to load plain dictionaries into main memory, you'll just want to compute some indexes (or something like that) after mapping.
However, the fact of the matter is that most of the dictionary will be read into memory anyway if it is available. If it is not available then there would be a good deal of disk swaps. Making characters 32-bit wide will increase the chance that there are more disk swaps. So the bottom line is that it is more efficient to convert characters from something like UTF-8 into some sort of 8-bit character. I could also use some sort of disk space lookup table such as the Berkeley Database. However this will definitely degrade performance.
The bottom line is that keeping Aspell 8-bit internally is a very well though out decision that is not likely to change any time soon. Feel free to challenge me on it, but, don't expect me to change my mind unless you can bring up some point that I have not thought of before and quite possibly a patch to solve cleanly convert Aspell to Unicode internally without a serious performance lost OR serious memory usage increase.