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, the extra symbols are bound to nil. 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. On the compiled backends a WASM program using progv compiles in exception-handling mode.