Indexing on runes is expensive, since either you need to store strings as arrays of runes or iterate on each index operation. Indexing on runes is also less useful than it may first seem. Consider that a rune is distinct from a glyph--a single "character" on the screen may be composed of several runes. The occasions when you care about specific runes as opposed to substrings are uncommon. When dealing with substrings, there is no advantage to substring-of-runes as opposed to substring-of-bytes.
Note that many languages that appear to offer indexing by rune (e.g., Java) do not in fact do so, since their 16-bit "character" type is incapable of representing all runes. The fact that this is only rarely an issue points at the fundamental rarity with with code needs to deal with runes-qua-runes.
Comments
Indexing on runes is expensive, since either you need to store strings as arrays of runes or iterate on each index operation. Indexing on runes is also less useful than it may first seem. Consider that a rune is distinct from a glyph--a single "character" on the screen may be composed of several runes. The occasions when you care about specific runes as opposed to substrings are uncommon. When dealing with substrings, there is no advantage to substring-of-runes as opposed to substring-of-bytes.
Note that many languages that appear to offer indexing by rune (e.g., Java) do not in fact do so, since their 16-bit "character" type is incapable of representing all runes. The fact that this is only rarely an issue points at the fundamental rarity with with code needs to deal with runes-qua-runes.