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4.3 Seqs and Strings ¶

Along with using heterogeneous trees to save space, the seq implementation tries to save even more space by using strings, when possible, for the short leaf vectors. This is possible when the contiguous sequence of up to 8 consecutive elements of the seq that happen to wind up in a particular leaf vector are all characters. In Lisp implementations where base-char is distinct from character, FSet will even use base-strings when possible. The upshot is:

• You can use seqs to hold character strings without worrying about space consumption.
• A seq can hold character strings interspersed with occasional non-characters quite efficiently; each non-character will force only one leaf vector to be a non-string. (Similarly, a seq can hold strings of base-chars interspersed with the occasional extended-char quite efficiently.)

Since seqs have log-time concatenation and subsequence operations, they can actually be very useful for string manipulation.