The change makes a difference because Typed Racket's type inference
for `cons` is producing a less-accurate answer than it could. When it
sees arguments of type `String` and `(Listof Any)`, it decides that
the result is a `(Listof Any)` instead of a `(Pairof String (Listof
Any))`.
If you add (excessive) annotation like this:
(apply fprintf (ann (cons (current-output-port) (ann (cons
message messageargs) (Pairof String (Listof Any))))
(Pairof Output-Port (Pairof String (Listof Any)))))
then the expression typechecks as written.
However, I think the use of `apply` without the extra `cons` calls is
clearer anyway, in addition to being easier for the type checker.
Sam
On Mon, Apr 13, 2020 at 3:22 PM K H <kieron.ha...@gmail.com> wrote:
>
> If I understand correctly, the difference to getting the code to type check
> was:
> > (apply fprintf (cons anomaly (cons message messageargs)))
>
> becomes:
> > (apply fprintf anomaly message messageargs)
>
> Since the documentation explains that the arguments to fprintf are
> effectively gathered in a list*, could someone explain why the change makes a
> difference? Would it the answer depend on whether messagesargs was a list?.
>
> Also, the first version with the cons's looks strange to me. Is there a
> reason why the statement was originally coded that way? Is it perhaps an
> artifact from attempting to force the result of list* to produce a list?
> regardless of what is the type of of the value passed in messageargs?
>
> Thanks in advance,
>
> Kieron
>
>
> On Sat, Apr 11, 2020 at 7:43 AM Hendrik Boom <hend...@topoi.pooq.com> wrote:
>>
>> On Fri, Apr 10, 2020 at 08:18:38PM -0400, Jon Zeppieri wrote:
>> > (define (unique [list : (Listof Any)] [message : String] .
>> > [messageargs : Any *])
>> > ; return the only element of the list, or '() if there is none.
>> > ; Produce message if not just one.
>> > (if (equal? 1 (length list)) (car list)
>> > (begin
>> > (apply fprintf anomaly message messageargs)
>> > (if (null? list) list (car list)))))
>>
>> Thank you. That worked.
>> The change to the apply syntax surprised me.
>>
>> -- hendrik
>>
>> >
>> > On Fri, Apr 10, 2020 at 7:49 PM Hendrik Boom <hend...@topoi.pooq.com>
>> > wrote:
>> > >
>> > > Trying to convert the following to typed Racket:
>> > >
>> > > (define (unique list message . messageargs)
>> > > ; return the only element of the list, or '() if there is none.
>> > > ; Produce message if not just one.
>> > > (if (equal? 1 (length list)) (car list)
>> > > (begin
>> > > (apply fprintf (cons anomaly (cons message messageargs)))
>> > > (if (null? list) list (car list))
>> > > )
>> > > )
>> > > )
>> > >
>> > > It's an error message function tat accepts a list to test.
>> > > If the test fails it uses fprintf to print a message with the
>> > > arguments for the ~s items.
>> > >
>> > > I got so far, but I don't know what to do with the . messageargs :
>> > >
>> > > (define (unique [list (Listof Any)] [message : String] . messageargs)
>> > > ; return the only element of the list, or '() if there is none.
>> > > ; Produce message if not just one.
>> > > (if (equal? 1 (length list)) (car list)
>> > > (begin
>> > > (apply fprintf (cons anomaly (cons message messageargs)))
>> > > (if (null? list) list (car list))
>> > > )
>> > > )
>> > > )
>>
>> --
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