D - Contract Programming

Contract programming in D programming is focused on providing a simple and understandable means of error handling.Contract programming in D are implemented by three types of code blocks:

• body block
• in block
• out block
  

body block

Body block contains the actual functionality code of execution. The in and out blocks are optional while the body block is mandatory. A simple syntax is shown below.

return_type function_name(function_params)
in
{
   // in block
}
out (result)
{
   // in block
}
body
{
   // actual function block
}

in block for pre conditions

In block is for simple pre conditions that verify whether the input parameters are acceptable and in range that can be handled by the code. A benefit of an in block is that all of the entry conditions can be kept together and separate from the actual body of the function. A simple precondition for validating password for its minimum length is shown below.

import std.stdio;
import std.string;

bool isValid(string password)
in
{
   assert(password.length>=5);
}
body
{
   // other conditions
   return true;
}

void main()
{
   writeln(isValid("password"));
}

When the above code is compiled and executed, it reads the file created in previous section and produces the following result:

true

out blocks for post conditions

The out block takes care of the return values from the function. It validates the return value is in expected range. A simple example containing both in and out is shown below that converts months, year to a combined decimal age form.

import std.stdio;
import std.string;

double getAge(double months,double years)
in
{
   assert(months >= 0);
   assert(months <= 12);
}
out (result)
{
   assert(result>=years);
}
body
{
   return years + months/12;
}

void main ()
{
   writeln(getAge(10,12));
}

When the above code is compiled and executed, it reads the file created in previous section and produces the following result:

12.8333