Clojure `case` doesn't differentiate different symbols

Question

This code works:

(case '-
    + :plus
    - :minus
    :unknown)
==> :minus

This too:

(case '-
    + :plus
    '- :minus
    * :times
    :unknown)
==> :minus

This doesn't:

(case '-
    '+ :plus
    '- :minus
    * :times
    :unknown)
==> java.lang.IllegalArgumentException: Duplicate case test constant: quote

Googling for this error leads to log file here. However, the guy just said it worked with quotes removed.

It looks like the case statement treats different symbols as the same value. Why is this so?

TIA.

Answer 1

From the case documentation:

Each clause can take the form of either:

test-constant result-expr

(test-constant1 ... test-constantN)  result-expr

'- expands to (quote -).

Therefore, the clauses in the case with quotes expand to:

(quote -) :minus
(quote +) :plus

As you can see, the symbol "quote" appears as an actual test constant, and when you quote both + and -, it appears twice.

Evaluating the case on the actual symbol quote might clarify things:

user=> (case 'quote '+ :plus - :minus :unknown)
:plus
user=> (case 'foo (foo +) :plus - :minus :unknown)
:plus