A vtable is a structure type, specifying its layout, and other information. A vtable is actually itself a structure, but there’s no need to worry about that initially (see Vtable Contents.)
Create a new vtable.
fields is a string describing the fields in the structures to be
created. Each field is represented by two characters, a type letter
and a permissions letter, for example
"pw". The types are as
p– a Scheme value. “p” stands for “protected” meaning it’s protected against garbage collection.
u– an arbitrary word of data (an
scm_t_bits). At the Scheme level it’s read and written as an unsigned integer. “u” stands for “unboxed”, as it’s stored as a raw value without additional type annotations.
The second letter for each field is a permission code,
w– writable, the field can be read and written.
r– read-only, the field can be read but not written.
Here are some examples.
(make-vtable "pw") ;; one writable field (make-vtable "prpw") ;; one read-only and one writable (make-vtable "pwuwuw") ;; one scheme and two unboxed
The optional print argument is a function called by
write (etc) to give a printed representation
of a structure created from this vtable. It’s called
(print struct port) and should look at struct and
write to port. The default print merely gives a form like
‘#<struct ADDR:ADDR>’ with a pair of machine addresses.
The following print function for example shows the two fields of its structure.
(make-vtable "prpw" (lambda (struct port) (format port "#<~a and ~a>" (struct-ref struct 0) (struct-ref struct 1))))