One of the claimed benefits of object-oriented programming is ENCAPSULATION.
The idea is that one of the ways a language helps you is by making some errors
difficult or impossible to express.
One of the things you need to understand an object is its INVARIANT.
For example, astc's SortedCollection includes
methods for: 'checking'
invariant
^(opposite isNil or: [
opposite isKindOf: DyadicBlock]) and: [
super invariant and: [
(sortBlock
ifNil:[(offset + 2 to: offset + size) allSatisfy: [:i |
(array at: i-1) <= (array at: i)]]
ifNotNil: [(sortBlock respondsTo: #value:value:) and: [
[(offset + 2 to: offset + size) allSatisfy: [:i |
(sortBlock value: (array at: i-1)
value: (array at: i)))]]
Private methods are allowed to break the invariant.
Public methods are not.
For example, consider reversing a SortedCollection.
Part of this changes the order of the elements, and can be
shared with OrderedCollection. Part of it swaps the arguments
of the sortBlock, and cannot be shared.
methods for: 'rearranging'
pvtPrereverse
|t|
opposite
ifNil: [
sortBlock ifNil: [sortBlock := Magnitude defaultSortBlock].
t := sortBlock.
opposite := sortBlock.
sortBlock := [:x :y | t value: y value: x]]
ifNotNil: [
t := sortBlock.
sortBlock := opposite.
opposite := t].
If this method could be called from "outside", it would certainly break
the invariant and leave the object in an unusable state.
Now a style rule doesn't *quite* prevent this method being called by
another object.
It is necessary to make sure that #perform: and friends cannot break
encapsulation
either, and this astc does by simply not creating Selector objects for
private methods.
Yes, it is a small increase in the complexity of the language, however
- you are not forced to write private methods
- if you run code that uses the pvt* convention in a Smalltalk that does
not use that convention, it works, it's just not safe any longer
- it is not an arbitrary addition to the language, it is a restriction that
makes programming *easier*.
Let's look at one additional example.
Behavior>>
new
^ self basicNew initialize
This means that *any* object can forcibly reinitialize *any* object it can
reach at *any* time. In astc, it's
new
"other checks"
^self pvtNew pvtPostNew
and an object cannot be reinitialised against its will.
There is a long standing tradition of "program for the typical case and trust
the omniscient programmer to know what will probably work and what won't"
in Smalltalk. This is why in many Smalltalks
aCollection addAll: aCollection
or
aCollection removeAll: aCollection
can go insane for some common collections. And one *DOES* get away
with it most of the time. It's typically when someone else triest to use your
code and doesn't know the assumptions you didn't bother to write down
that things go wrong. It's obvious that Pharo has got to where it
amazingly well is without the "bondage and discipline" of, say, Ada or
Agda. But there is a *reason* why Pharo has lint checking on by default.
On Fri, 20 Aug 2021 at 00:59, Jesus Mari Aguirre wrote:
>
> Please keep Pharo simple, why do you need private methods...you can include
> then in a protocol named private...other language have it...yes...next
> addition will be namespaces...other...I don't know...at last we have Java
>
> El jue., 19 ago. 2021 9:00, Richard O'Keefe escribió:
>>
>> Many years ago there was a proposal in the Squeak mailing list about
>> enforcing
>> a naming convention, "pvt", I implemented that in my Smalltalk system. The
>> compiler enforces the rule that pvt.[A-Z].* message can only be sent to
>> (self|super) (basicNew|basicNew: n|new|new: n|pvtNew: n)?
>> in a class method or
>> (self|super) ((class (new|new: n)|pvtSpeciesNew: n|pvtClone)?
>> in an instance method.
>> There are currently
>> 9412 public selectors
>> 793 pvt* selectors and
>>23 private* selectors,
>> where the last group is methods that I *want* to be private in some sense but
>> cannot do with this machinery. (For example, calling a "private" method on
>> another object known to be of the same class.)
>>
>> I think the evidence shows that this works well enough to be useful, even if
>> it
>> isn't quite as expressive as I'd like. And what *that* means is that
>> this can be
>> done with a style check, using the machinery Pharo already has for style
>> checks.
>>
>>
>>
>> On Wed, 18 Aug 2021 at 08:14, Craig Johnson wrote:
>> >
>> > Hi All,
>> >
>> >
>> > Just a newb off-the-wall question.
>> >
>> > Is there any good reason why we can't create a true private method in a
>> > Pharo class by putting that method inside an instance or class variable
>> > as a lambda (block).
>> >
>> >
>> > This would reduce one of my biggest
