uiop:symbol-call

(uiop:symbol-call package name &rest arguments)

UIOP's late-binding call: look name up in package at run time and apply it to arguments. Both are designators -- a keyword, a symbol or a string. This is how a library calls into a system it does not depend on and may not have loaded, which is why it is spelled with a run-time lookup rather than a direct call.

A package that does not exist, or a name that package does not have, signals -- the caller is about to apply the result, so an absent name is an error rather than a nil that fails one frame later.

It is also a function value, which is how the idiom is usually written: a library dispatching to one of several backends applies #'uiop:symbol-call to the arguments it was handed, and names the backend with uninterned symbols so that reading the form cannot require the package to exist.

Each arm of such a dispatch is resolved on its own, so an arm naming a backend this program does not have costs a call-time error if it ever runs, and nothing at all otherwise.

Backend support

• Interpreter: full support (the lookup runs against the live package and function tables).
• JVM and WASM: full support -- the call is late-bound through the compiled name registry, like funcall of a runtime-interned symbol, and #'uiop:symbol-call is a value like any other function. An absent package still signals; an absent name signals at the call (the undefined-function error) rather than at the lookup, slightly later than the interpreter's own probe but just as loud.