store/[org.clojure/clojure "1.9.0"] clj::clojure.core/set!

Official Documentation

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Usage
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   (set! var-symbol expr)
   (set! (. instance-expr instanceFieldName-symbol) expr)
   (set! (. Classname-symbol staticFieldName-symbol) expr)

Docstring
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  Used to set thread-local-bound vars, Java object instance
  fields, and Java class static fields.

Source

;; Special forms have no source

Example 1

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(set! var-symbol expr)

Example 2

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(set! *warn-on-reflection* true)