What does *allow-unresolved-vars* do in Clojure?

Question

Like there is no official document for it http://clojuredocs.org/clojure_core/clojure.core/allow-unresolved-vars

Answer 1

Apologies -- this is not a complete answer. I haven't figured out everything about it yet.

---

*allow-unresolved-vars* is defined in RT.java:

final static Var ALLOW_UNRESOLVED_VARS = Var.intern(CLOJURE_NS, Symbol.intern("*allow-unresolved-vars*"), F).setDynamic();

and used in Compiler.java:

    if(o == null)
        {
        if(RT.booleanCast(RT.ALLOW_UNRESOLVED_VARS.deref()))
            {
            return sym;
            }
        else
            {
            throw Util.runtimeException("Unable to resolve symbol: " + sym + " in this context");
            }
        }

So clearly, its use here is to decide whether or not an exception should be immediately thrown when an unresolved symbol is encountered.

You can mess with it like so:

myns.core=> (ns clojure.core)
nil

clojure.core=> oops!
CompilerException java.lang.RuntimeException: Unable to resolve symbol: oops! in this context, compiling:(/tmp/form-init1596111142512149454.clj:1:884) 

clojure.core=> (defn q [] (oops!))
CompilerException java.lang.RuntimeException: Unable to resolve symbol: oops! in this context, compiling:(/tmp/form-init1596111142512149454.clj:1:12) 

clojure.core=> (def *allow-unresolved-vars* true)
Warning: *allow-unresolved-vars* not declared dynamic and thus is not dynamically rebindable, but its name suggests otherwise. Please either indicate ^:dynamic *allow-unresolved-vars* or change the name. (/tmp/form-init1596111142512149454.clj:1)
#'clojure.core/*allow-unresolved-vars*

clojure.core=> oops!
IllegalArgumentException UnresolvedVarExpr cannot be evalled  clojure.lang.Compiler$UnresolvedVarExpr.eval (Compiler.java:1771)
clojure.core=> 

clojure.core=> (defn q [] (oops!))
CompilerException java.lang.VerifyError: (class: clojure/core$q, method: invoke signature: ()Ljava/lang/Object;) Unable to pop operand off an empty stack, compiling:(form-init1596111142512149454.clj:1:1) 

But I haven't figured out a use yet, because unresolved variables still cause errors -- they're just different errors. Also, I don't understand the warning when re-def-ing it, because the warning says that it's not declared dynamic, whereas it looks to me as though it is declared dynamic in RT.java.