[racket] Problem with case

On 9/17/12 6:47 PM, Kieron Hardy wrote:
> Hi all,
>
> Can anyone explain why the first function below selects 'x1-case as
> expected but the second fails to select 'x2-case? Am I expecting
> something to happen that shouldn't?
>
> Thanks,
>
> Kieron.
>
> ****
>
> #lang racket
>
> (case 'a
>     ['a 'x1-case]
>     ['b 'x1-case]
>     [(15 2 3) 'y1-case]
>     [(10 11 12) 'z1-case])
>
> (case 'a
>     [('a 'b) 'x2-case]
>     [(15 2 3) 'y2-case]
>     [(10 11 12) 'z2-case])

Be careful with quote in a case clause -- those clauses are already 
implicitly quoted.  The first is really:

(case 'a
   [(quote a) 'x1-case]
   [(quote b) 'x1-case]
   [(15 2 3) 'y1-case]
   [(10 11 12) 'z1-case])

So you'll notice the following surprising behavior:

(case 'quote
   ['a 'x1-case]
   ['b 'x1-case]
   [(15 2 3) 'y1-case]
   [(10 11 12) 'z1-case])

You want:

(case 'quote
   [(a) 'x1-case]
   [(b) 'x1-case]
   [(15 2 3) 'y1-case]
   [(10 11 12) 'z1-case])

and:

(case 'a
   [(a b) 'x1-case]
   [(15 2 3) 'y1-case]
   [(10 11 12) 'z1-case])

David