Simple string concatenation

I could not figure out how to concatenate list of strings. For example, I would like to modify the PS1 prompt variable by prepending a string. The function join is supposed to work on lists but in my experience it won't take a list as an input. So this for some reason works (join "," "item1" "item2") but this does not (join "," (list "item1" "item2")). It would also be helpful to have a function that splits a string by white space or by a separator passed as parameter and returns a list of strings. IMHO such and similar string manipulation functions would be quite useful while being just a generalization of what's already available for path manipulations.

Start by locating the join function and the list and string-manipulation entry points. Clarify whether the requested work includes accepting list arguments, splitting strings, or both, then define completion around the intended behavior for the PS1 example and separator handling.