design by contract versus doctest

[Colin Blackburn]

On Tue, 06 Apr 2004 12:41:45 +0100, Peter Hickman <peter at semantico.com>  
wrote:

> Looking at the Eiffel site the contract is enforced within the called  
> function.

Only while checking is turned on. It is a design philosophy. You specify  
the pre and post-conditions before code is even thought of, it is part of  
the design.

> put (x: ELEMENT; key: STRING) is
>      -- Insert x so that it will be retrievable through key.
>      require
>          count <= capacity
>          not key.empty
>      do
>          ... Some insertion algorithm ...
>      ensure
>          has (x)
>          item (key) = x
>          count = old count + 1
>      end

This says that *if* the pre-conditions are met (by the caller) *then* the  
post-conditions will be guaranteed to be met (by the called method.) All  
callers know this because the contract is publicly specified. New callers  
can make use of this method because they know what is required of them and  
that a given result will be assured if those requirements are met. They  
know this because the method has been designed correctly for these  
constraints. If the pre-conditions aren't met then not sensible result is  
guaranteed.

Colin
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