On Thu, Aug 30, 2018 at 12:20 PM, Matthew Butterick <m...@mbtype.com> wrote:
>
> On Aug 30, 2018, at 8:36 AM, David Storrs <david.sto...@gmail.com> wrote:
>
> I'd like to be able to write something like this:
>
> (match (hash 'a 1 'b 2)
> [(hash-table ('a a) ('b b) ('c c)) (list a b c)])
>
> ...except with something that says "if 'c isn't present, that's fine. Use
> this value instead."
>
>
>
> Perhaps move 'c out of the match pattern?
>
> (let ([h (hash 'a 1 'b 2)])
> (match h
> [(hash-table ('a a) ('b b))
> (list a b (hash-ref h 'c (λ () 'other-val)))]))
>
Yep. Or, alternatively, I could do it up front:
(define h (hash 'a 1))
(match (hash-set h 'b ((lambda (h) (hash-ref h 'b #f)) h))
[(hash-table ('a a) ('b b)) (list a b)])
=> '(1 #f)
That works, but it's pretty ugly.
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