In Elisp, how to get path string with slash properly inserted?

Question

I am manually constructing path strings in Elisp by concatenating partial paths and directory names. Unfortunately sometimes the paths end with slash, sometimes not. Theref

Answer 1

The easiest way to assemble file names from parts of questionable content is with expand-file-name. For example:

(expand-file-name "foo.txt")

this common form will give you a full file name based on default-directory:

/home/me/foo.txt

but if you have a variable 'dir' whose content is "/home/them/subdir" and want to use that, do this:

(expand-file-name "foo.txt" dir)

it doesn't matter if dir ends in / or not. If you are on some other platform, and contains the other slash, it will do the right thing then too. Do you have a mix? Just stack them:

(expand-file-name "foo.txt" (expand-file-name "somesubdir" dir))