How to test equality of quoted symbols in Scheme?

Question

In this example,

> (= 1 1)
#t
> (= 'a 'a)
*** ERROR IN (console)@2.1 -- (Argument 1) NUMBER expected
(= 'a 'a)
1> 

How can I test equality of quoted symbols?

Answer 1

You use any of eq?, eqv?, or equal?.

All three can be used with any objects without error (unlike =, which can only be used with numbers). However, the result will differ depending on which types you pass in. But if you know you're comparing symbols, all of them will have the same result.

If you have done any Java programming, eqv? is like ==, and equal? is like .equals(). In simple terms, eqv? does an identity comparison, and equal? does a value comparison.

(And eq? does a straight-up pointer comparison. For some implementations, it may be faster than eqv?, with the understanding that it sometimes returns false for equal numbers or characters. For other implementations, it's exactly identical to eqv?. Most of the time, for robustness, you should stick to using eqv? for doing identity comparisons, and forget that eq? exists.)