To pass by value or by reference?
On one hand, to get a "proper" dynamic development experience, we need to pass functions by reference:
(jetty/run-server #'app opts)On the other hand, using vars instead of functions has a performance impact, both because we pay for an extra deref, and derefing a var introduces a volatile read, which we might not want on our hot path.
We want to just pass app by value:
(jetty/run-server app opts)So what do we do? Abuse the compiler.
When we refer to functions in other functions, the compiler by default will pass them by reference, and the compiled function object will hold a reference to the Var, not the function object.
If we turn *direct-linking* on, function will be dereferenced at compile time, not run time.
So what happens if we eta-expand a function:
(defn foo [a] (inc a))
(def xf (map foo)) ;; pass foo by value
(def xf2 (map (fn [a] (foo a)))) ;; foo is passed by reference
(transduce xf conj [] (range 3)) ;; => [1 2 3]
(transduce xf2 conj [] (range 3)) ;; => [1 2 3]
;; Now redefine `foo` and use the transducers again
(defn foo [a] (+ a a))
(transduce xf conj [] (range 3)) ;; => [1 2 3]
(transduce xf2 conj [] (range 3)) ;; => [0 2 4] << SUCCESSAnd the cool part? If we use direct linking in production it will just get in-lined.
This library introduces a single macro, $, which eta expands to
multiple arities, allowing us to enjoy both worlds in development time
and production artifacts.
For further reading on this problem and how it affects development, refer to this excellent, thorough post by Joshua Suskalo
(require '[bsless.eta :refer [$]])
(jetty/run-server ($ app) opts)If you like being fancy, they all do the same:
(require '[bsless.eta :refer [$ η h]])Copyright © 2025 Ben Sless
Distributed under the Eclipse Public License version 1.0.