Re: syntax suggestion for concepts

"jam" <>
Tue, 6 Mar 2007 22:10:15 CST
On Mar 6, 6:38 pm, "Douglas Gregor" <> wrote:

On Mar 5, 7:55 pm, "W Karas" <> wrote:

On Mar 5, 1:02 pm, "Douglas Gregor" <> wrote:> On Mar 5, 9:12 am, "W Karas" <> wrote:

Making concept syntax more consistent with class interfaces would not
help programmers understand concepts any better. It may reduce initial
anxiety, because concepts would look like object-oriented interfaces,
but the resemblance would only be skin-deep. Concepts support a
different programming paradigm---Generic Programming---and much of the
confusion I've seen about concepts comes from attempts to think of
them in object-oriented terms. New ideas and constructs should have
new syntax, to emphasize that they are different from existing ideas/
constructs. Sometimes syntactic similarity can be a good thing, but
not when it leads to many incorrect assumptions.


You have made this argument repeatedly, but without a reference or
explaination as to why GP and OO are so distinct that it's
necessary to add otherwise unnecessary complexity to the syntax of
C++ to play up the difference.

There is a short write-up of the differences (at a feature level)

SmallTalk, unlike C++, does not have specific features to support
GP. But I have not seen or heard of a C++ template which cannot
be written in SmallTalk in an equally general way. Do you know
of such an example, or why this is not relevant?

Well, one obvious answer is any example of the Binary Method Problem.
Say you have some Shape interface and you want to be able to compare
Shapes with an isEqual method... I'll write it in Java, because I'm
less likely to mangle the syntax:

  interface Shape {
    boolean isEqual(Shape other);

Now, a Triangle is a Shape:

  class Triangle implements Shape {
      private int base;
      private int height;

      public boolean isEqual(Triangle other)
          return base == other.base && height == other.height;

In Java, this doesn't work and the compiler will complain because
Triangle's isEqual takes a Triangle whereas Shape's isEqual accepts a
shape. To get around Java's static type system, we use explicit casts
and write Triangle's isEqual like this:

    public boolean isEqual(Shape other_shape)
        Triangle other = (Triangle)other_shape;
        return base == other.base && height == other.height;

Similarly, we can define a Circle that also implements Shape and has
its own isEqual method. We can then trigger a run-time error by trying
to compare a Circle to a Triangle:

  Shape a = new Triangle(...);
  Shape b = new Circle(...);
  a.isEqual(b); // exception: run-time cast failed

Here's one of the earlier discussions of the binary method problem in
Smalltalk, which focuses on the double-dispatching issue: if you have
different kinds of graphical objects, and different kinds of display
ports, how can you write methods that display each kind of graphical
objects differently on each display port?

In the concepts world, Shape would be a concept with an isEqual
function, e.g.,

  concept Shape<typename T> {
    bool isEqual(T, T);

Note that we're using parametric polymorphism to describe concepts,
not subtype polymorphism: that's problem the single, most important
theoretical difference between OO and GP.

To say that Triangle is a Shape, we write an externally-defined
concept map:

  concept map Shape<Triangle> {
    bool isEqual(Triangle a, Triangle b) {
      return a.base == b.base && a.height == b.height;

We would do the same for Circle.

The static type system prevents us from ever getting into trouble with
isEqual: Triangle meets the requirements of Shape, and Circle meets
the requirements of Shape, but that does not imply anything about the
relationship between Triangle and Circle. For example:

  template<typename T>
  where Shape<T>
  bool equal_shapes(T a, T b) {
    return isEqual(a, b);

  Triangle a;
  Circle b;
  equal_shapes(a, b); // compile-time error: T=Triangle in first arg,
T=Circle in second arg.

Parametric polymorphism, as in templates, retains the identity of
types. Subtype polymorphism, as in object-oriented languages, loses
type identity. To write the equivalent of the Java Shape's isEqual
method with parametric polymorphism in a constrained template, we'd
effectively be saying:

  template<typename T, typename U>
  where Shape<T> && Shape<U>
  bool equal_shapes2(T a, U b) {
    return isEqual(a, b);

The real types T and U vary independently, but both are Shapes. When
this is done in a OO world, the type-checking is pushed to run time,
inside the body of isEqual (whose interface says that *any* two shapes
can be compared). In a GP world (and with C++ concepts), the isEqual
call inside equal_shapes2 would fail at compile time: the Shape
concept says that a T can be compared with itself, rather than saying
that it can be compared against any Shape.

If you want to say that any Shape can be compared against any other
Shape (say, in a hasSameArea function), you would use more parametric

  concept ShapeWithArea<typename T> : Shape<T> {
    double area(T);

    template<typename U> where Shape<U> bool hasSameArea(T, U);


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Do you mean that refinement syntax is not orignated from that of
inheritance and will not confuse every one?

- It again mixes object-oriented and generic programming ideas in a
way that will lead to confusion. I guarantee that, after showing an
example like the above to an audience, I will get two questions: (1)
why can't I just use ':' to inherit from C? and (2) can I declare a
variable of type C*? Neither question would arise without the
assumption that concepts are just another take on object-oriented
programming (they aren't), and we need to be very careful not to give
the impression that they are.

enerything has a price .For innovation yuo have to be braver than
this. Is the fear that programers might misunderstand a reason for us
not to do anything?.
but there exists one reason for not using concept keyword in that
manor (if I am not mistaken) :concepts allow use of a type when
afeature is available and accesible and ban otherwise but what we need
here is that we want to remind ourselves not to forget to provide a
specific interface for a class currently declared but not defined.I
suggest the 'interface' keyword syntax similar to that of 'concept'
for this porpuse that unlike JAVA will not necessarily mean a runtime
polymorphism.I would like to wrote:

interface CC<typename T>{
 void go();
 virtual void gogo();

CC struct my_type{
/*definition of following functons is a must now and copy-ctor must
not be banned(private or protected).
 private: void go();
private: virtual void gogo();

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