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
follows.
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.
It used to be that the second letter for each field was a permission
code, such as w for writable or r for read-only. However
over time structs have become more of a raw low-level facility; access
control is better implemented as a layer on top. After all,
struct-set! is a cross-cutting operator that can bypass
abstractions made by higher-level record facilities; it’s not generally
safe (in the sense of abstraction-preserving) to expose
struct-set! to “untrusted” code, even if the fields happen to
be writable. Additionally, permission checks added overhead to every
structure access in a way that couldn’t be optimized out, hampering the
ability of structs to act as a low-level building block. For all of
these reasons, all fields in Guile structs are now writable; attempting
to make a read-only field will now issue a deprecation warning, and the
field will be writable regardless.
(make-vtable "pw") ;; one scheme field (make-vtable "pwuwuw") ;; one scheme and two unboxed fields
The optional print argument is a function called by
display and 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 "pwpw"
(lambda (struct port)
(format port "#<~a and ~a>"
(struct-ref struct 0)
(struct-ref struct 1))))