A rope is a standard data structure that represents a single string as a tree of strings, allowing concatenation to do no work up front.
That is, a string formed by many Rope concatenations followed by a to_string needs
only copy each input to the output once, whereas a string expression looking like
a ^ b ^ c ^ ... ^ z must create an intermediate string for every
concatenation, and will copy the original data into and out of short-lived
temporary strings many times.
On the other hand, because String.concat [ s1; s2; s3; ... ] allocates a single
string and copies the inputs into it, Rope is no improvement over that usage.
Rope becomes useful when the construction of the sequence of strings is more
complex -- a good example is prettyprinting an expression language, where you need to
parenthesize subexpressions (appending a short string at both ends) and handle infix
binary operators (appending two long strings both made up of many parts, with a short
string in between).
Any operations that would produce a Rope longer than String.max_length raise
instead. They are not marked with _exn on their names since (at least on 64-bit)
this number is far in excess of the size of your memory, so isn't likely to come up in
practice.
A more fully-featured implementation of the same idea is available in the zed
library.