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

[Olivier Dion via General Guile related discussions <guile-user@gnu.org>]

On Fri, 05 Mar 2021, Tim Meehan <btmeehan@gmail.com> wrote:
> 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"))))))

You've quoted the whole s-exp.  Which means lambda is never applied.
You have to apply lambda in order to create a procedure.

>
> (define res (hash-ref stuff 'a))
> (res)
>
> But when I run it:
> Wrong type to apply: (lambda () (display "event a\n"))

You're trying to apply a list and not a procedure.  This is because of
the quote mentioned above.  In other words, you're doing this:
----------------------------------------------------------------------
(apply '(lambda () (display "event a\n")))
----------------------------------------------------------------------

Change the ' for ` and unquote the lambdas with ,.  Like this:
----------------------------------------------------------------------
(define stuff (alist->hash-table
  `((a . ,(lambda () (display "event a\n")))
    (b . ,(lambda () (display "event b\n")))
    (c . ,(lambda () (display "event c\n"))))))
----------------------------------------------------------------------

Or use the list procedure instead of quoting:
----------------------------------------------------------------------
#! /usr/bin/guile
!#

(use-modules (ice-9 hash-table))

(define stuff (alist->hash-table
  (list (cons 'a (lambda () (display "event a\n")))
        (cons 'b (lambda () (display "event b\n")))
        (cons 'c (lambda () (display "event c\n"))))))

(define res (hash-ref stuff 'a))
(res)
----------------------------------------------------------------------

-- 
Olivier Dion
PolyMtl