Re: syntax suggestion for concepts

"W Karas" <>
Tue, 6 Mar 2007 22:09:38 CST
On Mar 6, 9:38 am, "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);


I feel what you are saying here can be summarized as: When using
templates, names are bound to specific types at compile time. When
using OO, names may not be bound to specific types until runtime.
Unsurprisingly, any binding errors will occur when the binding occurs.

Late binding is generally bad for performance and (I and probably you
and others would agree) for understandability. But it's more
Generic algorithms meant for heterogeneous types work better with late
bindings, a benefit which is the mirror image of the "problem" you
show above.

Another way to address multiple dispatch in a way more similar to
C++ OO is to think of the function as a "member" of an implied
class that is the tuple of the types of the function parameters.

Is ultimately the key difference between GP and OO the fact
that member functions are central to OO but not to GP? To
me, member functions seem more central to encapsulation than
polymorphism. It seems easy to conceive of a programming
language with runtime polymorphism but no membership, where
vptrs are passed as hidden function paramaters rather than
being hidden object data members. With this approach,
multiple dispatch becomes easy -- there is a vptr hidden
parameter for each "polymorphic type tuple" rather than
just for each polymorphic type.

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