Editing Eiffel (programming language) (section)

===Genericity===
{{See also|Generic programming#Genericity in Eiffel}}

A generic class is a class that varies by type (e.g. LIST [PHONE], a list of phone numbers; ACCOUNT [G->ACCOUNT_TYPE], allowing for ACCOUNT [SAVINGS] and ACCOUNT [CHECKING], etc.). Classes can be generic, to express that they are parameterized by types. Generic parameters appear in square brackets:

<syntaxhighlight lang="eiffel">
class LIST [G] ...
</syntaxhighlight>

G is known as a "formal generic parameter". (Eiffel reserves "argument" for routines, and uses "parameter" only for generic classes.) With such a declaration G represents within the class an arbitrary type; so a function can return a value of type G, and a routine can take an argument of that type:

<syntaxhighlight lang="eiffel">
item: G do ... end
put (x: G) do ... end
</syntaxhighlight>

The <code>LIST [INTEGER]</code> and <code>LIST [WORD]</code> are "generic derivations" of this class. Permitted combinations (with <code>n: INTEGER</code>, <code>w: WORD</code>, <code>il: LIST [INTEGER]</code>, <code>wl: LIST [WORD]</code>) are:

<syntaxhighlight lang="eiffel">
n := il.item
wl.put (w)
</syntaxhighlight>

<code>INTEGER</code> and <code>WORD</code> are the "actual generic parameters" in these generic derivations.

It is also possible to have 'constrained' formal parameters, for which the actual parameter must inherit from a given class, the "constraint". For example, in

<syntaxhighlight lang="eiffel">
   class HASH_TABLE [G, KEY -> HASHABLE]
</syntaxhighlight>

a derivation <code>HASH_TABLE [INTEGER, STRING]</code> is valid only if <code>STRING</code> inherits from <code>HASHABLE</code> (as it indeed does in typical Eiffel libraries). Within the class, having <code>KEY</code> constrained by <code>HASHABLE</code> means that for <code>x: KEY</code> it is possible to apply to <code>x</code> all the features of <code>HASHABLE</code>, as in <code>x.hash_code</code>.