Except maybe for one thing: I wonder if the version of case defined here
is written in such a way that is smart in that it never has to make said
hash table / alist more than once, at compile time. I'm guessing so?
Christopher Lemmer Webber writes:
> As I started typing this email and looking into the definition of case,
> I realized my assumptions are wrong.
>
> What I needed: something like case which dispatches on symbols, except
> not auto-quoting the arguments... I needed to evaluate them from the
> lexical environment. So:
>
> (let ([x 'foo])
> (caseish 'foo
> [(x) 'got-foo]
> [('x) 'got-quote-x])) ; => 'got-foo
>
> I figured: case is fast, and I'm pretty sure semi-magical... my
> intuitive sense was that it did some smart things on a compiler level
> that would probably be anything I'd hand-code (which would either use an
> alist or an immutable hashtable). So I started writing up an email
> asking if such a thing worked... then I remembered, this is a ~lisp,
> jumping straight to definition is part of the scene... so I did that.
>
> I... was wrong! From the case macro:
>
> ;; Symbol and "other" dispatch is either sequential or
> ;; hash-table-based, depending on how many constants we
> ;; have. Assume that `alist' does not map anything to `#f'.
> (define (dispatch-hashable tmp-stx alist make-hashX else-exp)
> (if (< (length alist) *hash-threshold*)
> #`(case/sequential #,tmp-stx
> #,@(map (λ (x)
> #`[(#,(car x)) #,(cdr x)])
> alist)
> [else #,else-exp])
> (let ([tbl (make-hashX alist)])
> (if (literal-expression? else-exp)
> #`(hash-ref #,tbl #,tmp-stx (lambda () #,else-exp))
> #`(or (hash-ref #,tbl #,tmp-stx (lambda () #f))
> #,else-exp)))))
>
> Am I reading that right? Here I was assuming writing something like
> case was for cool kids and I'd never pull such a thing off right.
>
> But... now it looks like, oh, it's actually just using an alist or an
> immutable hashtable... same as I would.
>
> So I almost trashed this email, but thought I'd send it anyway because
> a) either maybe it's interesting to someone or b) actually maybe I'm
> misreading and case really is smarter / more performant and I'm just
> missing it...!
>
> - Chris
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