Can't I use symbols in case?

Questions & Answers
1

Check out this small snippet from the REPL:

> (define (foo p) (case p (('bar) 1) (else 2)))
> (foo 'bar)
2

I expected to the call (foo 'bar) would return 1. Why is 'bar not matched?

2

Well, the short version here is: don't use case, it's a very old form and has surprising conventions. Specifically, in this case, the elements in the matching clauses are automatically quoted, so you probably wanted to write:

> (define (foo p) (case p ((bar) 1) (else 2)))
> (foo 'bar)
1
3

What would you have me use instead of case? The closest thing would probably be cond; however I prefer case in cases when I have values to match, because the cond clauses are more verbose in this case.

4

Usually it is better to use match than case:

> (define (foo p)
    (match p
      ['bar 1]
      [_ 2]))
> (foo 'bar)
1

(One very niche reason when you actually might want case instead of match is when you care about the guarantee that “the case form can dispatch to a matching case-clause in O(log N) time for N datums.”)

5

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Note the (datum ...). This means you don't need the quote before the symbol.

The reader will turn 'bar into (quote bar), so

(case p (('bar) 1) ...)

is the same as

(case p ( ((quote bar)) 1) ...)

so the list of the datums in the case clause does not include a lone bar symbol.