A Clojure library is a source file used to store and reuse code. It declares a namespace whose name reflects the file’s relative path: path separators become dots. The file must be available on the classpath for `require` to find it. section 16.0 section 17.3
A library and a namespace are closely related, but they are not interchangeable. Loading a library creates its namespace; creating a namespace alone does not create a file that can be loaded. `require` loads a library and its transitive dependencies, while `loaded-libs` reports libraries already loaded. section 16.0 section 16.10
Load a library
`(require 'clojure.set)` makes the `clojure.set` namespace available. Use an alias, such as `:as se`, when you want access without referring every symbol into the current namespace; `:refer :all` can replace existing mappings and produce warnings. section 16.10
(create-ns 'test-require) ; #1 (require 'test-require) ; #2 ;; Could not locate test_require__init.class or test_require.clj on classpath.
Creating `test-require` as a namespace does not make it a library: `require` still fails because it cannot locate a corresponding file on the classpath. section 16.10
For direct evaluation, `load` reads from a classpath-relative path, while `load-file` can evaluate a file outside the classpath. `load-string` evaluates code supplied as a string and, unlike `read-string` followed by `eval`, handles multiple forms without a `do` wrapper and retains line numbers in var metadata. section 17.3
