Re: Explain error

[Aaron Hill]

On 2019-11-08 7:12 pm, Freeman Gilmore wrote:
> \version "2.19.83"
> #((define h (make-hash-table 114))
> (display h)
> (hashq-set! h 3 "val3")
> (display (hashq-ref h 3)))
>
> Wrong type to apply: #<unspecified>
>
> Could someone explain why I get this error?

"(define h (make-hash-table 114))" returns (or evaluates to) 
#<unspecified> which is not a procedure.  (#<unspecified> is a special 
value in Scheme, which you can test for with the "unspecified?" 
predicate.)

It helps to remember that "(proc 1 2 3)" is shorthand for "(apply proc 
'(1 2 3))".  Your original expression, therefore, is functionally 
equivalent to this:

;;;;
(apply (define h (make-hash-table 114))
  (list (display h)
        (hashq-set! h 3 "val3")
        (display (hashq-ref h 3))))
;;;;

The solution is to use "begin":

%%%%
\version "2.19.83"
#(begin
  (define h (make-hash-table 114))
  (display h)
  (hashq-set! h 3 "val3")
  (display (hashq-ref h 3)))
%%%%


-- Aaron Hill