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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