progv

(progv symbols values body...)

Evaluates symbols and values (each a list), then dynamically binds each symbol to the corresponding value for the duration of body, restoring the previous values on exit. When values is shorter than symbols, each extra symbol is bound with no value: for the extent of body it is unbound, so reading it signals an unbound-variable naming it and boundp answers nil, and an assignment gives the binding a value. On the compiled backends a standard variable (*print-base*, *standard-output*, ...) is bound to nil there instead. Unlike let, the symbols are computed at runtime and need not have been proclaimed special. Returns the value of the last body form.

progv runs on all backends. The compilers lower it to a dispatch over the program's statically known special variables, so a symbol that is a special of the program gets a true dynamic binding; any other symbol is bound so that symbol-value and boundp see it for the extent. Since any special can be among the symbols, a compiled program that uses progv tests every read of a special variable for a missing value. On the compiled backends a WASM program using progv compiles in exception-handling mode.