Common Lisp assignment: which expression correctly assigns the current value of a to b (i.e., after evaluation, b will equal a’s value)?

Introduction / Context: Understanding assignment syntax is essential in Lisp, which uses prefix notation and special forms for variable binding and mutation. The task is to set variable b to the value currently held in a.

Given Data / Assumptions:

• Dialect: Common Lisp.
• Goal: after evaluation, b has the same value as a.
• We are not creating a local binding; we are assigning.

Concept / Approach: The special form setq performs assignment using the pattern (setq var value). Therefore, (setq b a) copies the value of a into b. Forms like (b = a) or (set b = a) reflect C-like or pseudo-code syntax and are invalid in Lisp. (setq a b) would reverse the assignment direction, setting a to b, not vice versa.

Step-by-Step Solution:

Verification / Alternative check: In a REPL: set a first (e.g., (setq a 42)), then evaluate (setq b a); now b evaluates to 42.

Why Other Options Are Wrong:

Common Pitfalls: Mixing up setq versus local binding with let, and reversing source/destination order.

Final Answer: (setq b a)