Editing Eiffel (programming language) (section)

===Conversions===
Eiffel provides a mechanism to allow conversions between various types. The mechanisms coexists with inheritance and complements it. To avoid any confusion between the two mechanisms, the design enforces the following principle:

: '''(Conversion principle) A type may not both conform and convert to another.'''

For example, <code>NEWSPAPER</code> may conform to <code>PUBLICATION</code>, but <code>INTEGER</code> converts to <code>REAL</code> (and does not inherit from it).

The conversion mechanism simply generalizes the ad hoc conversion rules (such as indeed between <code>INTEGER</code> and <code>REAL</code>) that exist in most programming languages, making them applicable to any type as long as the above principle is observed. For example, a <code>DATE</code> class may be declared to convert to <code>STRING</code>; this makes it possible to create a string from a date simply through

<syntaxhighlight lang="eiffel">
   my_string := my_date
</syntaxhighlight>

as a shortcut for using an explicit object creation with a conversion procedure:

<syntaxhighlight lang="eiffel">
   create my_string.make_from_date (my_date)
</syntaxhighlight>

To make the first form possible as a synonym for the second, it suffices to list the creation procedure (constructor) <code>make_from_date</code> in a <code>convert</code> clause at the beginning of the class.

As another example, if there is such a conversion procedure listed from <code>TUPLE [day: INTEGER; month: STRING; year: INTEGER]</code>, then one can directly assign a tuple to a date, causing the appropriate conversion, as in

<syntaxhighlight lang="eiffel">
      Bastille_day := [14, "July", 1789]
</syntaxhighlight>