Re: Somehow I have got this "thunk" thing wrong.

[Tim Meehan]

Also thanks Olivier and Aleix, and now I will have to watch that talk as
well.

On Fri, Mar 5, 2021 at 9:40 PM Tim Meehan <btmeehan@gmail.com> wrote:

> Yes, that makes sense. The unquote is a neat way to fix that.
> One of these days, I will learn my lesson.
> Thanks Christopher!
>
> On Fri, Mar 5, 2021 at 6:26 PM Christopher Baines <mail@cbaines.net>
> wrote:
>
>>
>> Tim Meehan <btmeehan@gmail.com> writes:
>>
>> > I wanted to store a thunk in a hashtable so that I could look up its key
>> > and then run it later. Something like this:
>> >
>> > #! /usr/bin/guile
>> > !#
>> >
>> > (use-modules (ice-9 hash-table))
>> >
>> > (define stuff (alist->hash-table
>> >   '((a . (lambda () (display "event a\n")))
>> >     (b . (lambda () (display "event b\n")))
>> >     (c . (lambda () (display "event c\n"))))))
>> >
>> > (define res (hash-ref stuff 'a))
>> > (res)
>> >
>> > But when I run it:
>> > Wrong type to apply: (lambda () (display "event a\n"))
>>
>> The lambda bit you've written is quoted. So you're asking Guile to apply
>> a list where the first element is the symbol 'lambda, the second is the
>> empty list, ...
>>
>> You probably want something like this, where you're creating a list of
>> pairs, where the car of the pair is a symbol, and the cdr is a procedure
>> (rather than a list).
>>
>>  (define stuff (alist->hash-table
>>    `((a . ,(lambda () (display "event a\n")))
>>      (b . ,(lambda () (display "event b\n")))
>>      (c . ,(lambda () (display "event c\n"))))))
>>
>> Does that make sense?
>>
>