@node YAOS Primitives
@section Primitives

What Scheme lacks is a proper object system. This library solves that
problem @emph{once and for all}!

The records created through this library are all immutale, where each
field becomes a multiple dispatch procedure, whence called with a
single argument returns the value of that field, and when called with
two values (first one being the object, second being the new value)
returns a new instance of the object, with the field in question
replaced.

@defmac define-type (name type-parameters ...) fields ...
Introduce a new type, disjoint from all previously defined types.

@c Each field is either a symbol, or a list where the first element is a
@c symbol, and the remaining elements are alternating keywords and
@c values, as per @ref{Field Parameters}. All fields are optional by
@c default, but can be made non-optional through its type parameter.

@c The example below creates a new type called @var{type}, with a custom
@c printer which always displays the string ``TYPE''. It has two fields,
@c @var{x}, which must be an integer, and @var{y}, which can have any
@c type, but gets the value ``Hello'' in none is given.
@c @example
@c (define-type (type #:printer (lambda (r p) (display "TYPE" p)))
@c   (x #:type integer?)
@c   (y #:default "Hello"))
@c @end example

@var{type-parameters} is a key-value list, where the valid keys are

@deffn {Type Parameter} #:constructor (λ (primitive-constructor type-validator))
The defaulct constructor for these types simply takes a keyword-value
list, validates each argument in regard to the defined types, and
constructs a final object. However; sometimes a more advanced
constructor is needed, which can be added through this parameter.

The custom constructor is called with two values:

@itemize
@item the types primitive (and usually hidden) constructor,
which takes as many arguments as there are fields, in the order given
in define-type, and
@item the type validator procedure, which also takes all arguments,
but instead either returns an undefined value if everything is fine,
or throws @code{'wrong-type-arg} otherwise.
@end itemize
The procedure should then return a new procedure, which will be bound
as the constructor for the type. Note that default values are current
disregarded with custom constructors.

A custom constructor for the type above might look like
@example
(lambda (primitive-constructor type-check)
  (lambda* (#:key x y)
    (type-check x y)
    (primitive-constructor x y)))
@end example
@end deffn

@deffn {Type Parameter} #:printer (λ (record port))
Use a custom printer for the type.
@end deffn

Each field declares a part of the record, along with relevant metadata
about that field. The most basic declaration is a bare symbol, which
adds a field which can hold anything. However, a list with the target
name as its first element can instead be given, with the tail
containing keyword arguments as follows

@deffn {Field Parameter} default value
Value the field should get if not given.
@end deffn

@deffn {Field Parameter} type type-clause
A type predicate that the field must obey. See @ref{type-clause} for details.

Each type introduces a number of bindings, which are@footnote{
@var{<name>} here refers to the name of the type
}:
@end deffn

@defun @var{<name>} [kv-args ...]
Type constructor. Takes key-value arguments. Where the keys are the
names of the fields.
@end defun

@defun @var{<name>}? x
Type predicate.
@end defun

And for each field @var{<field>}:

@defun @var{<field>} object [value]
Accessor for the given filed.
Returns the current value if called with only an object, and returns a
new object with @var{field} set to @var{value} if called with two values.

The updating version checks the type if #:type was given on creation.
@end defun

@end defmac