Difference between with-local-vars and with-bindings in Clojure

clojure, dynamic-binding

Solution

New `var`s are temporarily created by `with-local-vars`. Existing `var`s are temporarily rebound by `with-bindings`. In both cases the bindings are thread-local.

Note that `with-bindings` is, as far as I can tell, primarily useful as a helper to pass bindings from another context by using a map returned by `get-thread-bindings`. The similar function `binding` would be more typical when not importing bindings.

Illustrative examples:

(binding [*out* (new java.io.StringWriter)] 
  (print "world!") (str "hello, " *out*))
;=> "hello, world!"

(with-local-vars [*out* (new java.io.StringWriter)] 
  (print "world!") (str "hello," *out*))
;=> world!"hello,#<Var: --unnamed-->"

(with-local-vars [foo (new java.io.StringWriter)] 
  (.write @foo "world") (str "hello, " @foo))
;=> "hello, world"

(binding [foo (new java.io.StringWriter)] 
  (.write @foo "world") (str "hello, " @foo))
;=> CompilerException java.lang.RuntimeException: 
;     Unable to resolve var: foo in this context...

Problem

Original source