Over the years, a few programming paradigms have been successful enough to enter the casual vocabulary of software engineers: procedural, imperative, object-oriented, functional, generic, declarative. There's a B-list, too, that includes paradigms such as logic, constraint-oriented, and symbolic. The point is, there aren't very many of them altogether.

Design by Introspection is a proposed programming paradigm that is at the same time explosively productive and firmly removed from any of the paradigms considered canon. The tenets of Design by Introspection are:

1. The rule of optionality: Component primitives are almost entirely opt-in. A given component is required to implement only a modicum of primitives, and all others are optional. The component is free to implement any subset of the optional primitives.
2. The rule of introspection: A component user employs introspection on the component to implement its own functionality using the primitives offered by the component.
3. The rule of elastic composition: A component obtained by composing several other components offers capabilities in proportion with the capabilities offered by its individual components.

Let's start with a small refresher.

Templates are the feature that allows describing the code as a pattern, for the compiler to generate program code automatically. Parts of the source code may be left to the compiler to be filled in until that part is actually used in the program. Templates are very useful especially in libraries because they enable writing generic algorithms and data structures, instead of tying them to specific types.

static if is the compile time equivalent of the if statement. Just like the if statement, static if takes a logical expression and evaluates it. Unlike the if statement, static if is not about execution flow; rather, it determines whether a piece of code should be included in the program or not. The logical expression must be evaluable at compile time. If the logical expression evaluates to true, the code inside the static if gets compiled. If the condition is false, the code is not included in the program as if it has never been written.

Let's assume we want to define a use function that can use an object of type T to get, deliver and optionally wrap another object (we'll use an int for simplicity). The code for such a function will look something like the following

void use(T)(T object) {
  // ...
  int v = object.get();
  // Compute stuff on v
  object.wrap(v);
  // ...
  object.deliver();
  // ...
}

The code above uses compile-time polymorphism. Compile-time polymorphism requires that the type is compatible with how it is used by the template. As long as the code compiles, the template argument can be used with that template.

As you can see, we don't fully respect the contract, as the use of wrap is not optional. To make it optional, we'll add a static if that checks if T has a wrap method.

Q: How can we do this? A: Simple. Let's just ask the compiler if code that calls object.wrap(v) is compilable. This is easily done in the D programming language through it's traits features.

Traits are extensions to the language to enable programs, at compile time, to get at information internal to the compiler. This is also known as compile time reflection. The D programming language has a comprehensive set of ready to use traits.

In our example, we are interested in the compiles trait.

Have a look at the updated code:

void use(T)(T object) {
  // ...
  int v = object.get();
  // Compute stuff on v
  static if (__traits(compiles, { object.wrap(42); }) {
    object.wrap(v);
  }
  // ...
  object.deliver();
  // ...
}

Now we respect the contract: we only call the wrap method with an int if T defines one that is callable with an int.

We can also use the is expression to achieve the same compile time check, with the following idiom:

  static if (is(typeof({ object.wrap(42); }))) {
    object.wrap(v);
  }

Though it might look complicated, it's not. What happens here is that typeof({ object.wrap(42); }) would be a compile-time error if we can't call wrap. A compile time error is not a valid type, so the result of is(error) is false. Go ahead, try the two static ifs out.

Now, we respect our contract, but we can do better. If a user tries to call use with a type that doesn't have get or deliver, he will get a compile time error, but the error will come from the library function instead of the user call site. To fix this, we need to add a template constraint

void use(T)(T object)
if (is (typeof(object.get())) &&
    is (typeof(object.deliver())))
{
  /* ... */
}

Although such constraints achieve the desired goal, sometimes they are too complex to be readable. Instead, it is possible to give a more descriptive name to the whole constraint:

void use(T)(T object)
if (canGetAndDeliver!T)
{
  /* ... */
}

That constraint is more readable because it is now more clear that the template is designed to work with types that can get and deliver. Such constraints are achieved by an idiom that is implemented similar to the following eponymous template:

template canGetAndDeliver(T) {
  enum canGetAndDeliver = is (typeof(
  {
    T object;
    object.get();
    object.deliver();
  }()));
}

Mixins are for mixing in generated code into the source code. The mixed in code may be generated as a template instance or a string .

Template mixins insert instantiations of templates into the code by the mixin keyword:

mixin a_template!(template_parameters)

As we will see in the example below, the mixin keyword is used in the definitions of template mixins as well.

The instantiation of the template for the specific set of template parameters is inserted into the source code right where the mixin keyword appears.

For example, let's have a template that defines both an array of edges and a pair of functions that operate on those edges:

mixin template EdgeArrayFeature(T, size_t count) {
  T[count] edges;
  void setEdge(size_t index, T edge) {
    edges[index] = edge;
  }
  void printEdges() {
    writeln("The edges:");
    foreach (i, edge; edges) {
      writef("%s:%s ", i, edge);
    }
    writeln();
  }
}

That template leaves the type and number of array elements flexible. For example, the mixin below can insert the two-element int array and the two functions that are generated by the template right inside a struct definition:

struct Line {
  mixin EdgeArrayFeature!(int, 2);
}

As a result, Line ends up defining a member array and two member functions:

import std.stdio;
 
void main() {
  auto line = Line();
  line.setEdge(0, 100);
  line.setEdge(1, 200);
  line.printEdges();
}

Another powerful feature of D is being able to insert code as string as long as that string is known at compile time. The syntax of string mixins requires the use of parentheses:

mixin (compile_time_generated_string)

For example, the hello world program can be written with a mixin as well:

import std.stdio;
 
void main() {
  mixin (`writeln("Hello, World!");`);
}

Obviously, there is no need for mixins in these examples, as the strings could have been written as code as well.

The power of string mixins comes from the fact that the code can be generated at compile time.

A common use case of string mixins is Operator Overloading:

struct A {
  int x;
 
  ref A opOpAssign(string op)(ref A rhs)
  if (op == "+" || op == "-")
  {
    mixin("this.x" ~ op ~ "= rhs.x;");
  }
}
 
void main() {
  A a1 = A(20);
  A a2 = A(22);
 
  a1 += a2;
  writeln(a1); // -> 42
  a1 -= a2;
  writeln(a1); // -> 20

Another example is represented by string predicates. Let's consider the following function template that takes an array of numbers and returns another array that consists of the elements that satisfy a specific condition:

int[] filter(string predicate)(int[] numbers) {
  int[] result;
  foreach (number; numbers) {
    if (mixin (predicate)) {
      result ~= number;
    }
  }
  return result;
}

That function template takes the filtering condition as its template parameter and inserts that condition directly into an if statement as is.

For that condition to choose numbers that are e.g. less than 7, the if condition should look like the following code:

if (number < 7) {

The users of the filter() template can provide the condition as a string:

int[] numbers = [ 1, 8, 6, -2, 10 ];
int[] chosen = filter!"number < 7"(numbers);

Importantly, the name used in the template parameter must match the name of the variable used in the implementation of filter() . So, the template must document what that name should be and the users must use that name.

Phobos uses names consisting of single letters like a, b, n, etc.

We want to define a struct CheckedInt that defines facilities for efficient checking of integral operations against overflow, casting with loss of precision, unexpected change of sign, etc. CheckedInt must work with both built-in integral types and other CheckedInts.

CheckedInt offers encapsulated integral wrappers that do all checking internally and have configurable behavior upon erroneous results. For example, CheckedInt!int is a type that behaves like int but aborts execution immediately whenever involved in an operation that produces the arithmetically wrong result.

The declaration should look something like

struct Checked(T, Hook)

In order to work with built-in types, you need to define: opAssign, opBinary, opBinaryRight, opCast, opCmp, opEquals, opOpAssign, opUnary.

CheckedInt has customizable behavior with the help of a second type parameter, Hook. Depending on the Hook type, core operations on the underlying integral may be verified for overflow or completely redefined. Implement the following predefined hooks:

1. Abort : fails every incorrect operation with a message to std.stdio. stderr followed by a call to assert(0). It is the default second parameter, i.e. Checked!short is the same as Checked!(short, Abort).
2. Throw : fails every incorrect operation by throwing an exception.
3. Warn: prints incorrect operations to std.stdio.stderr but otherwise preserves the built-in behavior.