FullName is always of the same type (string or atom) as RelName.
Success: [eclipse]: absolute_file_name("file", Full). Full = "/homes/tom/file" yes [eclipse]: absolute_file_name(file, Full). Full = '/homes/tom/file' yes [eclipse]: absolute_file_name("~/file", Full). Full = "/homes/tom/file" yes [eclipse]: absolute_file_name(library(fd), Full). Full = '/usr/local/eclipse/lib/fd.sd' yes Error: absolute_file_name(X, Y). (Error 4). absolute_file_name(4, X). (Error 5).