Earlier quoted context omitted.
"Choosing the equal sign to denote assignment is one notoriously bad example that goes back to Fortran in 1957 and has been copied blindly by armies of language designers since. This bad idea overthrows a century-old tradition to let = denote a comparison for equality, a predicate that is either true or false. But Fortran made this symbol mean assignment, the enforcing of equality. In this case, the operands are on u…
Lisp, the second oldest language after Fortran, didn't make this mistake. In Lisp, "=" isn't an operator at all. Assignment is (setf x y) and comparison is (eq x y) [and several variants of eq].
Common Lisp and related languages have = as a function: it is a numeric comparison. Like under equalp, (= 1 1.0) yields true. You might think it's superfluous to equalp, but has some virtues of its own, like supporting variadic arguments (= 1 1 1 1) and rejecting non-numeric arguments.
eq is often the wrong one to use: two instances of the same number can fail to be eq.