When to use which type of STRING in eiffel? I saw using READABLE_STRING_GENERAL and having to l_readable_string.out' to convert it to STRING`
READABLE_STRING_GENERAL is an ancestor of all variants of strings: mutable, immutable, 8-bit, 32-bit, so it can be used as a formal argument type when the feature can handle any string variant.
READABLE_STRING_32 is a good choice when the code handles Unicode, and can work with either mutable or immutable versions.
STRING_32 is a mutable Unicode variant. The code can change its value.
STRING is an alias for a string type that can either be STRING_8 or STRING_32. At the time of writing only a few libraries are adapted to handle the mapping of STRING to STRING_32. However, this mapping could become the default in the future to facilitate working with Unicode.
Regardless of the future, I would recommend using ..._STRING_32 to process strings. This way the code directly supports Unicode. The libraries are also updated in this direction. For example, io.put_string_32 can be used to print a Unicode string to the standard output (using the current locale).