The threading package is a set of threading macros for Common Lisp. While the implementation differs, this code took heavy inspiration from Racket's threading library: https://docs.racket-lang.org/threading/index.html.
Threading is the process of passing return values from one function to the next. For example:
CL-USER > (~> 10 (- 20) (+ 3))
-7
The initial value was passed as the head parameter to the expanded forms of (- 20) and (+ 3). The above would be equivalent to:
CL-USER > (+ (- 10 20) 3)
-7
If you'd like the value to be threaded as the tail parameter, use ~>> instead:
CL-USER > (~>> 10 (- 20) (+ 3))
13
The above would be equivalent to:
CL-USER > (+ 3 (- 20 10))
13
The following macros are defined by the threading package:
(~> x &rest forms)
(~>> x &rest forms)
Additionally, there are two more macros:
(and~> x &rest forms)
(and~>> x &rest forms)
These are identical except that - if at any time one of the forms returns nil - threading ceases and nil is returned.
That's it!