boundp

(boundp symbol)

Returns t when symbol names a bound dynamic variable -- one with a global value (defvar/defparameter/top-level setq) or an active dynamic binding (a let of a special variable, a parameter named like one, progv) -- nil otherwise. Like Common Lisp's boundp, lexical bindings (a let of an ordinary variable, function parameters) are invisible. t, nil and keywords are self-evaluating constants, so boundp of any of them is t.

On the compiled backends a boundp of a LITERAL symbol is answered at compile time and costs nothing when the definitions before the call decide it: a compiled program cannot make a global appear at run time. A variable no definition gives a value -- a special variable declared without one, or a name only a function body or a later top-level setq assigns -- is answered at run time by its variable, without the eval runtime. A computed symbol ((boundp (intern name))) is resolved at run time too, and pulls the embedded eval runtime into the output like eval does — as symbol-value and fboundp do whatever their argument is. A program compiled with --dynamic, or one that calls eval/load, keeps the run-time check throughout.

As a function value (#'boundp, as in (mapcar #'boundp names)) it is a computed probe on every backend: the answer comes at run time, and the eval runtime is pulled in only for a program that names #'boundp.

A special variable declared without a value is bound only for the extent of a binding of it: