You can write Lisp that is <i>de facto</i> statically typed, and good compilers take advantage of it. If we write (cons a b), that piece of program text has a type, which is a cons.  Given (let ((c (cons a b))) ... (car c) ...) the Lisp compiler can generate efficient code to access c, without a run-time type check that c is a cons, based on c inheriting the type from the cons expression, and not being subject to any assignment in its scope. If we regard it as a parametrized type, not knowing what a and b is, it is the expression &quot;for all types x, for all types y, the type of (cons x y) is: cons cell of x and y&quot;.