declare

(declare declaration...)

A declaration other than special never changes what a program computes: the whole declare form evaluates to nil and its arguments are never evaluated or validated, so any standard declaration (ignore, ignorable, type, optimize, inline, special, ...) is accepted anywhere in a body, and source code written for other Common Lisp implementations loads unchanged.

Three declaration families do affect compilation. A (declare (special ...)) at the head of a body makes a binding of a listed variable that the form establishes dynamic, and the references to it in that body read the dynamic binding, as in standard CL; an inner binding of the same name is lexical unless it is declared special too. On the WASM backend, a type declaration that names an array type -- (simple-array (unsigned-byte 8) (*)), simple-vector, simple-string, ... -- lets the compiler emit that one representation's element accessors directly, which makes the compiled module smaller and faster. And on the JVM backend, a type declaration that names a float type -- double-float, single-float, or float -- keeps the declared local variables in raw double slots and compiles arithmetic over them as unboxed machine instructions, which makes the compiled class faster, especially before the JIT warms up. Results are unaffected by a correct declaration; a false one, which is undefined behavior in Common Lisp, traps at the access on WASM and signals a catchable type error at the declared read or store on the JVM, while the interpreter continues to ignore it.