java-lang-boolean-cannot-be-cast-to-clojure-lang-symbol.md

ClassCastException java.lang.Boolean cannot be cast to clojure.lang.Symbol

Description

You have attempted to call symbol on something other than a string or symbol (in this case a boolean). symbol ultimately depends on the intern method of clojure.lang.Symbol.java, which will only accept a string argument (symbol arguments to symbol are special-cased to return themselves). Note that symbol will accept arguments that result in non-conformant symbols (eg (symbol "ab cd")), but it is highly recommend that you not do so.

Cause 1: Forgetting brackets around a require clause

Forgetting brackets around a require clause causes this error. For example:

(ns error-sandbox.core
  (:require clojure.walk :as walk))

A clue that this is the cause of the exception is seeing clojure.core/load-lib in the stack trace.

Solution 1

The above should instead be:

(ns error-sandbox.core
  (:require [clojure.walk :as walk]))

Cause 2: General case

There can of course be other causes, as simple as

(symbol true)

Solution 2

Don't try to call symbol on anything but a string.