Re: Extending functional call syntax for Sequences to include Maps

[Jamison Hope <jhope@alum.mit.edu>]

On Fri, Jul 3, 2020 at 6:05 PM Per Bothner <per@bothner.com> wrote:
>
> On 7/3/20 2:24 PM, Duncan Mak via Kawa wrote:
> > More concisely, you can use (Kawa-specific) function call syntax:
> >
> > (vec1 3) ⇒ 8
> >
> >
> > I was a little disappointed when I found out that this doesn't work for
> > Maps, which is also something Clojure allows [0]:
> >
> > ;; This is Clojure not Scheme
> >
> > ({:age 42 :name "Joe"} :name)     ; ⇒ "Joe"
> > ({:age 42 :name "Joe"} :age)      ; ⇒ 42
> > ({:age 42 :name "Joe"} :unknown)  ; ⇒ nil
>
> I vaguely remember discussing this some years ago, and I think the main
> (conceptual) problem is how to handle arguments with no binding, i.e.
> the 3rd case above.  By analogy with sequences, it should throw an
> exception - which may not be the most convenient
>
> I mentioned a couple of possible approaches, but I don't recall
> what they were.  I think I was leaning towards a way to specify
> a default value - and throw an exception when no default was specified.
>
> I haven't been able to find that email, so far.

I suspect you're remembering this conversation we had five (!) years ago:
https://sourceware.org/pipermail/kawa/2015q1/006284.html

You were leaning toward throwing an exception on no-default, and I was
leaning toward returning #!null for simplicity/consistency with Java,
and also because #!null is treated as false, which is like what
assq/assv/assoc and memq/memv/member return for an absent key.

-J

>
> A possibly solution:
>
> (htab key [default])
>
> If there is no entry for key in htab, then default is returned
> if specified; otherwise an exception is thrown.
>
> You should also be able to do:
>
> (set! (htab key) new-value)
> --
>         --Per Bothner
> per@bothner.com   http://per.bothner.com/



-- 
Jamison Hope