If you have a chain of single-argument functions and you want to do something
side-effectful in the middle of the chain with just the input at that point in
the chain, you can make a function that converts a void-returning function into
a function that returns its argument:
(define ((side-effect proc) v)
(proc v)
v)
(~> 10
(* 2)
(side-effect displayln) ;; prints 20, then passes 20 down the chain
(+ 5))
;; returns 25
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