[pro] When to use SLOT-VALUE...

[Daniel Weinreb]

You considered the idea that when you write a library (internal
module, whatever) that defines a new type of Lisp object, it should
not necessarily be apparent to the caller whether the implementation
of objects of that type happen to use CLOS.

    Hans: Of course, one may want to argue that DEFCLASS forms should not
    be considered to be an interface definition. Instead, one could call
    for a series of DEFGENERIC forms to define the external interface of
    some "module" and make class definitions be internal.

In addition to this, you'd use a factory function, rather than having
the client call make-instance, to hide the CLOS nature of the type.

I agree with you about this, except that I would go farther and use
packages.

In fact, I wrote a library in this style, called fhash.  It creates an
abstraction that implements a map, just like Common Lisp hash-tables,
except that internally it uses linear search when the number of items
is small, and upgrades to to a real hash table when the number of
items gets large.

This is very much like what Hans is talking about, with one slight
difference.  In the fhash library, I took the position that Common
Lisp's mechanism for encapsulation is NOT CLOS, not object-oriented
programing, but packages.

The packge declaration says:

  (:export
   #:fhash-table
   #:make-fhash-table
   #:getfhash
   #:putfhash
   #:remfhash
   #:clrfhash
   #:mapfhash
   #:fhash-table-p
   #:fhash-table-count
   #:fhash-table-size
   #:fhash-table-test
   #:with-fhash-table-iterator)

More about why using packages is right, in subsequent
mail, since that could turn into a separate thread.

-- Dan


Hans Hübner wrote:
>
> Hi,
>
> The company I work for has a Common Lisp style guide that generally 
> disallows using SLOT-VALUE.  Instead, accessor should be used so that: 
> BEFORE and :AFTER methods are always invoked when accessing a slot.  
> Generally, I think this is a good idea when looking at classes and 
> instances from the outside.  Slots should be considered as being an 
> implementation detail, and users (i.e. client code and derived class 
> methods) should not make assumptions about how functionality is 
> implemented.
>
> Now, I often have the need for class instances that are constant in 
> some respect, i.e. some properties of the instance that are 
> implemented using slots can't directly be changed.  I often declare 
> such slots havin only a: READER in the class definition, which makes 
> the read-only nature of this slot apparent right away.
>
> Of course, such slots need to be initialized somehow. An :INITARG 
> sometimes does the trick, but it is more common that the value of such 
> slots is calculated and recalculated during the lifetime of the 
> instance, and as such the slot's value must be set.
>
> Now, from the perspective of seeing the class declaration as 
> documenting the visible behavior of instances of a class, it does not 
> seem to be proper to declare an accessor to be used in class-internal 
> code so that the slot's value can be updated.  Instead, I think that 
> it is better to use SLOT-VALUE to mess with the guts of an instance 
> from code that is part of the guts itself.
>
> Of course, one may want to argue that DEFCLASS forms should not be 
> considered to be an interface definition. Instead, one could call for 
> a series of DEFGENERIC forms to define the external interface of some 
> "module" and make class definitions be internal. From a more practical 
> perspective, though, class definitions in CL serve both as interface 
> and implementation definition, thus it seems to be appropriate using 
> the mechanisms provided by CLOS to support both uses.
>
> How do others use or avoid SLOT-VALUE? Is it frowned upon in your 
> company's or project's (verbal) style guide?
>
> Thanks for your input,
> Hans
>
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