Common Lisp assignment semantics: Which function evaluates both the variable argument and the value expression when assigning, thereby allowing indirect (symbol-valued) assignment?
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Aset
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Bsetq
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Cadd
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Deva
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ENone of the above
Answer
Correct Answer: set
Explanation
Introduction / Context:In Common Lisp, variable assignment can be done with multiple forms. Understanding when the variable name is evaluated versus treated as a literal symbol is crucial, especially for indirect assignments and metaprogramming tasks.
Given Data / Assumptions:
- We compare
setandsetqsemantics. - We care about whether the first argument (the variable) is evaluated.
- Goal: identify the form that evaluates both its arguments.
Concept / Approach:setq assigns to a named variable without evaluating the variable name; only the value expression is evaluated. By contrast, set evaluates its first argument to obtain a symbol and evaluates its second argument for the new value. Thus, set supports indirect assignment when the symbol to assign is itself computed.
Step-by-Step Solution:
Consider (setq x 10): ‘‘x’’ is not evaluated; value 10 is stored in variable x.Consider (set (read-from-string "x") 10): first argument evaluates to the symbol X; second evaluates to 10; X is then assigned 10.Therefore, the function that evaluates both variable and value isset.Select ‘‘set’’ as the correct choice.Verification / Alternative check:Test in a REPL: (let ((s 'x')) (set (symbol-value 's') 5)) demonstrates evaluated first arg; (setq s 5) would instead set variable s, not X.
Why Other Options Are Wrong:
setqdoes not evaluate the variable name.addand ‘‘eva’’ are not standard Common Lisp assignment primitives.
Common Pitfalls:Passing a raw symbol to set without quoting or ensuring evaluation yields a symbol leads to errors; ensure the first argument evaluates to a symbol object.
Final Answer:set