View source for Template:Computer Programming/Error Handling/6

As you see the function demands a precondition of <code>X >= 0</code> - that is the function can only be called when <var>X</var> &ge; 0. In return the function promises as postcondition that the return value is also &ge; 0.

In a full DbC approach, the postcondition will state a relation that fully describes the value that results when running the function, something like <code>result &ge; 0 and X = result * result</code>. This postcondition is √'s part of the contract. The use of assertions, annotations, or a language's type system for expressing the precondition <code>X >= 0</code> exhibits two important aspects of Design by Contract:

# There can be ways for the compiler, or analysis tool, to help check the contracts.  (Here for example, this is the case when <code>X &ge; 0</code> follows from X's type, and √'s argument when called is of the same type, hence also &ge; 0.)
# The precondition can be mechanically checked '''before''' the function is called.

The 1<sup>st</sup> aspect adds to safety: No programmer is perfect. Each part of the contract that needs to be checked by the programmers themselves has a high probability for mistakes.

The 2<sup>nd</sup> aspect is important for optimization &mdash; when the contract can be checked at compile time, no runtime check is needed. You might not have noticed but if you think about it: <math>A^2 + B^2</math> is never negative, ''provided'' the exponentiation operator and the addition operator work in the usual way. 

We have made 5 nice error handling examples for a piece of code which never fails. And this is the great opportunity for controlling some runtime aspects of DbC: You can now safely turn checks off, and the code optimizer can omit the actual range checks.

DbC languages distinguish themselves on how they act in the face of a contract breach:

# True DbC programming languages combine DbC with exception handling &mdash; raising an exception when a contract breach is detected at runtime, and providing the means to restart the failing routine or block in a known good state.
# Static analysis tools check all contracts at analysis time and demand that the code written in such a way that no contract can ever be breached at runtime.
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