<s>
Eiffel	B-Language
is	O
an	O
object-oriented	B-Language
programming	I-Language
language	I-Language
designed	O
by	O
Bertrand	O
Meyer	O
(	O
an	O
object-orientation	O
proponent	O
and	O
author	O
of	O
Object-Oriented	B-Language
Software	O
Construction	O
)	O
and	O
Eiffel	B-Language
Software	O
.	O
</s>
<s>
In	O
2005	O
,	O
Eiffel	B-Language
became	O
an	O
ISO-standardized	O
language	O
.	O
</s>
<s>
The	O
design	O
of	O
the	O
language	O
is	O
closely	O
connected	O
with	O
the	O
Eiffel	B-Language
programming	I-Language
method	O
.	O
</s>
<s>
Both	O
are	O
based	O
on	O
a	O
set	O
of	O
principles	O
,	O
including	O
design	B-Application
by	I-Application
contract	I-Application
,	O
command	B-Application
–	I-Application
query	I-Application
separation	I-Application
,	O
the	O
uniform-access	O
principle	O
,	O
the	O
single-choice	O
principle	O
,	O
the	O
open	O
–	O
closed	O
principle	O
,	O
and	O
option	B-Application
–	I-Application
operand	I-Application
separation	I-Application
.	O
</s>
<s>
Many	O
concepts	O
initially	O
introduced	O
by	O
Eiffel	B-Language
later	O
found	O
their	O
way	O
into	O
Java	B-Language
,	O
C#	B-Application
,	O
and	O
other	O
languages	O
.	O
</s>
<s>
New	O
language	O
design	O
ideas	O
,	O
particularly	O
through	O
the	O
Ecma/ISO	O
standardization	O
process	O
,	O
continue	O
to	O
be	O
incorporated	O
into	O
the	O
Eiffel	B-Language
language	O
.	O
</s>
<s>
The	O
key	O
characteristics	O
of	O
the	O
Eiffel	B-Language
language	O
include	O
:	O
</s>
<s>
An	O
object-oriented	B-Language
program	O
structure	O
in	O
which	O
a	O
class	O
serves	O
as	O
the	O
basic	O
unit	O
of	O
decomposition	O
.	O
</s>
<s>
Design	B-Application
by	I-Application
contract	I-Application
tightly	O
integrated	O
with	O
other	O
language	B-Language
constructs	I-Language
.	O
</s>
<s>
Automatic	B-General_Concept
memory	I-General_Concept
management	I-General_Concept
,	O
typically	O
implemented	O
by	O
garbage	B-General_Concept
collection	I-General_Concept
.	O
</s>
<s>
Inheritance	B-Language
,	O
including	O
multiple	B-Application
inheritance	I-Application
,	O
renaming	O
,	O
redefinition	B-Language
,	O
"	O
select	O
"	O
,	O
,	O
and	O
other	O
mechanisms	O
intended	O
to	O
make	O
inheritance	B-Language
safe	O
.	O
</s>
<s>
A	O
uniform	O
type	O
system	O
handling	O
both	O
value	O
and	O
reference	O
semantics	O
in	O
which	O
all	O
types	O
,	O
including	O
basic	O
types	O
such	O
as	O
INTEGER	O
,	O
are	O
class-based	B-Application
.	O
</s>
<s>
Void	B-Application
safety	I-Application
,	O
or	O
static	O
protection	O
against	O
calls	O
on	O
null	O
references	O
,	O
through	O
the	O
attached-types	O
mechanism	O
.	O
</s>
<s>
Agents	O
,	O
or	O
objects	O
that	O
wrap	O
computations	O
,	O
closely	O
connected	O
with	O
closures	B-Language
and	O
lambda	B-Language
calculus	I-Language
.	O
</s>
<s>
Keyword-based	O
syntax	O
in	O
the	O
ALGOL/Pascal	O
tradition	O
but	O
separator-free	O
,	O
insofar	O
as	O
semicolons	O
are	O
optional	O
,	O
with	O
operator	O
syntax	O
available	O
for	O
routines	O
.	O
</s>
<s>
Simple	O
Concurrent	B-Application
Object-Oriented	I-Application
Programming	I-Application
(	O
SCOOP	B-Application
)	O
facilitates	O
creation	O
of	O
multiple	O
,	O
concurrently	O
active	O
execution	O
vehicles	O
at	O
a	O
level	O
of	O
abstraction	O
above	O
the	O
specific	O
details	O
of	O
these	O
vehicles	O
(	O
e.g.	O
</s>
<s>
Eiffel	B-Language
emphasizes	O
declarative	O
statements	O
over	O
procedural	O
code	O
and	O
attempts	O
to	O
eliminate	O
the	O
need	O
for	O
bookkeeping	O
instructions	O
.	O
</s>
<s>
Eiffel	B-Language
shuns	O
coding	O
tricks	O
or	O
coding	O
techniques	O
intended	O
as	O
optimization	O
hints	O
to	O
the	O
compiler	B-Language
.	O
</s>
<s>
Eiffel	B-Language
's	O
simplicity	O
is	O
intended	O
to	O
promote	O
simple	O
,	O
extensible	O
,	O
reusable	O
,	O
and	O
reliable	O
answers	O
to	O
computing	O
problems	O
.	O
</s>
<s>
Compilers	B-Language
for	O
computer	O
programs	O
written	O
in	O
Eiffel	B-Language
provide	O
extensive	O
optimization	O
techniques	O
,	O
such	O
as	O
automatic	O
in-lining	O
,	O
that	O
relieve	O
the	O
programmer	O
of	O
part	O
of	O
the	O
optimization	O
burden	O
.	O
</s>
<s>
Eiffel	B-Language
was	O
originally	O
developed	O
by	O
Eiffel	B-Language
Software	O
,	O
a	O
company	O
founded	O
by	O
Bertrand	O
Meyer	O
.	O
</s>
<s>
Object-Oriented	B-Language
Software	O
Construction	O
contains	O
a	O
detailed	O
treatment	O
of	O
the	O
concepts	O
and	O
theory	O
of	O
the	O
object	B-Language
technology	I-Language
that	O
led	O
to	O
Eiffel	B-Language
's	O
design	O
.	O
</s>
<s>
The	O
design	O
goal	O
behind	O
the	O
Eiffel	B-Language
language	O
,	O
libraries	O
,	O
and	O
programming	O
methods	O
is	O
to	O
enable	O
programmers	O
to	O
create	O
reliable	O
,	O
reusable	O
software	O
modules	O
.	O
</s>
<s>
Eiffel	B-Language
supports	O
multiple	B-Application
inheritance	I-Application
,	O
genericity	B-Language
,	O
polymorphism	B-Application
,	O
encapsulation	B-Application
,	O
type-safe	O
conversions	O
,	O
and	O
parameter	O
covariance	O
.	O
</s>
<s>
Eiffel	B-Language
's	O
most	O
important	O
contribution	O
to	O
software	B-General_Concept
engineering	I-General_Concept
is	O
design	B-Application
by	I-Application
contract	I-Application
(	O
DbC	B-Application
)	O
,	O
in	O
which	O
assertions	O
,	O
preconditions	O
,	O
postconditions	O
,	O
and	O
class	B-Application
invariants	I-Application
are	O
employed	O
to	O
help	O
ensure	O
program	O
correctness	O
without	O
sacrificing	O
efficiency	O
.	O
</s>
<s>
Eiffel	B-Language
's	O
design	O
is	O
based	O
on	O
object-oriented	B-Language
programming	I-Language
theory	O
,	O
with	O
only	O
minor	O
influence	O
of	O
other	O
paradigms	O
or	O
concern	O
for	O
support	O
of	O
legacy	O
code	O
.	O
</s>
<s>
Eiffel	B-Language
formally	O
supports	O
abstract	O
data	O
types	O
.	O
</s>
<s>
Under	O
Eiffel	B-Language
's	O
design	O
,	O
a	O
software	O
text	O
should	O
be	O
able	O
to	O
reproduce	O
its	O
design	O
documentation	O
from	O
the	O
text	O
itself	O
,	O
using	O
a	O
formalized	O
implementation	O
of	O
the	O
"	O
Abstract	O
Data	O
Type	O
"	O
.	O
</s>
<s>
EiffelStudio	B-Application
is	O
an	O
integrated	B-Application
development	I-Application
environment	I-Application
available	O
under	O
either	O
an	O
open	B-Application
source	I-Application
or	O
a	O
commercial	O
license	O
.	O
</s>
<s>
It	O
offers	O
an	O
object-oriented	B-Language
environment	O
for	O
software	B-General_Concept
engineering	I-General_Concept
.	O
</s>
<s>
EiffelEnvision	O
is	O
a	O
plug-in	O
for	O
Microsoft	B-Application
Visual	I-Application
Studio	I-Application
that	O
allows	O
users	O
to	O
edit	O
,	O
compile	B-Language
,	O
and	O
debug	O
Eiffel	B-Language
projects	O
from	O
within	O
the	O
Microsoft	B-Application
Visual	I-Application
Studio	I-Application
IDE	O
.	O
</s>
<s>
Five	O
other	O
open	B-Application
source	I-Application
implementations	O
are	O
available	O
:	O
"	O
The	O
Eiffel	B-Language
Compiler	B-Language
"	O
tecomp	O
;	O
Gobo	O
Eiffel	B-Language
;	O
SmartEiffel	B-Application
,	O
the	O
GNU	O
implementation	O
,	O
based	O
on	O
an	O
older	O
version	O
of	O
the	O
language	O
;	O
LibertyEiffel	B-Application
,	O
based	O
on	O
the	O
SmartEiffel	B-Application
compiler	B-Language
;	O
and	O
Visual	B-Application
Eiffel	I-Application
.	O
</s>
<s>
Several	O
other	O
programming	O
languages	O
incorporate	O
elements	O
first	O
introduced	O
in	O
Eiffel	B-Language
.	O
</s>
<s>
Sather	B-Application
,	O
for	O
example	O
,	O
was	O
originally	O
based	O
on	O
Eiffel	B-Language
but	O
has	O
since	O
diverged	O
,	O
and	O
now	O
includes	O
several	O
functional	B-Language
programming	I-Language
features	O
.	O
</s>
<s>
The	O
interactive-teaching	O
language	O
Blue	O
,	O
forerunner	O
of	O
BlueJ	B-Language
,	O
is	O
also	O
Eiffel-based	O
.	O
</s>
<s>
The	O
Apple	B-Application
Media	I-Application
Tool	I-Application
includes	O
an	O
Eiffel-based	O
Apple	O
Media	O
Language	O
.	O
</s>
<s>
The	O
Eiffel	B-Language
language	O
definition	O
is	O
an	O
international	O
standard	O
of	O
the	O
ISO	O
.	O
</s>
<s>
The	O
standard	O
was	O
developed	O
by	O
ECMA	O
International	O
,	O
which	O
first	O
approved	O
the	O
standard	O
on	O
21	O
June	O
2005	O
as	O
Standard	O
ECMA-367	B-Language
,	O
Eiffel	B-Language
:	O
Analysis	O
,	O
Design	O
and	O
Programming	O
Language	O
.	O
</s>
<s>
Eiffel	B-Language
Software	O
,	O
"	O
The	O
Eiffel	B-Language
Compiler	B-Language
"	O
tecomp	O
and	O
Eiffel-library-developer	O
Gobo	O
have	O
committed	O
to	O
implementing	O
the	O
standard	O
;	O
Eiffel	B-Language
Software	O
's	O
EiffelStudio	B-Application
6.1	O
and	O
"	O
The	O
Eiffel	B-Language
Compiler	B-Language
"	O
tecomp	O
implement	O
some	O
of	O
the	O
major	O
new	O
mechanisms	O
—	O
in	O
particular	O
,	O
inline	O
agents	O
,	O
assigner	O
commands	O
,	O
bracket	O
notation	O
,	O
non-conforming	O
inheritance	B-Language
,	O
and	O
attached	O
types	O
.	O
</s>
<s>
The	O
SmartEiffel	B-Application
team	O
has	O
turned	O
away	O
from	O
this	O
standard	O
to	O
create	O
its	O
own	O
version	O
of	O
the	O
language	O
,	O
which	O
they	O
believe	O
to	O
be	O
closer	O
to	O
the	O
original	O
style	O
of	O
Eiffel	B-Language
.	O
</s>
<s>
Object	O
Tools	O
has	O
not	O
disclosed	O
whether	O
future	O
versions	O
of	O
its	O
Eiffel	B-Language
compiler	B-Language
will	O
comply	O
with	O
the	O
standard	O
.	O
</s>
<s>
LibertyEiffel	B-Application
implements	O
a	O
dialect	O
somewhere	O
in	O
between	O
the	O
SmartEiffel	B-Application
language	O
and	O
the	O
standard	O
.	O
</s>
<s>
The	O
standard	O
cites	O
the	O
following	O
,	O
predecessor	O
Eiffel-language	O
specifications	O
:	O
</s>
<s>
Bertrand	O
Meyer	O
:	O
Object-Oriented	B-Language
Software	O
Construction	O
,	O
Prentice	O
Hall	O
:	O
first	O
edition	O
,	O
1988	O
;	O
second	O
edition	O
,	O
1997	O
.	O
</s>
<s>
An	O
Eiffel	B-Language
"	O
system	O
"	O
or	O
"	O
program	O
"	O
is	O
a	O
collection	O
of	O
classes	O
.	O
</s>
<s>
Above	O
the	O
level	O
of	O
classes	O
,	O
Eiffel	B-Language
defines	O
cluster	O
,	O
which	O
is	O
essentially	O
a	O
group	O
of	O
classes	O
,	O
and	O
possibly	O
of	O
subclusters	O
(	O
nested	O
clusters	O
)	O
.	O
</s>
<s>
Clusters	O
are	O
not	O
a	O
syntactic	O
language	B-Language
construct	I-Language
,	O
but	O
rather	O
a	O
standard	O
organizational	O
convention	O
.	O
</s>
<s>
Typically	O
an	O
Eiffel	B-Language
program	O
will	O
be	O
organized	O
with	O
each	O
class	O
in	O
a	O
separate	O
file	O
,	O
and	O
each	O
cluster	O
in	O
a	O
directory	O
containing	O
class	O
files	O
.	O
</s>
<s>
A	O
class	O
contains	O
features	O
,	O
which	O
are	O
similar	O
to	O
"	O
routines	O
"	O
,	O
"	O
members	O
"	O
,	O
"	O
attributes	O
"	O
or	O
"	O
methods	O
"	O
in	O
other	O
object-oriented	B-Language
programming	I-Language
languages	I-Language
.	O
</s>
<s>
Eiffel	B-Language
's	O
standard	O
data	O
types	O
,	O
such	O
as	O
INTEGER	O
,	O
STRING	O
and	O
ARRAY	O
,	O
are	O
all	O
themselves	O
classes	O
.	O
</s>
<s>
Eiffel	B-Language
has	O
five	O
basic	O
executable	O
instructions	O
:	O
assignment	O
,	O
object	O
creation	O
,	O
routine	O
call	O
,	O
condition	O
,	O
and	O
iteration	O
.	O
</s>
<s>
Eiffel	B-Language
's	O
control	O
structures	O
are	O
strict	O
in	O
enforcing	O
structured	O
programming	O
:	O
every	O
block	O
has	O
exactly	O
one	O
entry	O
and	O
exactly	O
one	O
exit	O
.	O
</s>
<s>
Unlike	O
many	O
object-oriented	B-Language
languages	I-Language
,	O
but	O
like	O
Smalltalk	B-Application
,	O
Eiffel	B-Language
does	O
not	O
permit	O
any	O
assignment	O
into	O
attributes	O
of	O
objects	O
,	O
except	O
within	O
the	O
features	O
of	O
an	O
object	O
,	O
which	O
is	O
the	O
practical	O
application	O
of	O
the	O
principle	O
of	O
information	B-Application
hiding	I-Application
or	O
data	O
abstraction	O
,	O
requiring	O
formal	O
interfaces	B-Application
for	O
data	O
mutation	O
.	O
</s>
<s>
To	O
put	O
it	O
in	O
the	O
language	O
of	O
other	O
object-oriented	B-Language
programming	I-Language
languages	I-Language
,	O
all	O
Eiffel	B-Language
attributes	O
are	O
"	O
protected	O
"	O
,	O
and	O
"	O
setters	O
"	O
are	O
needed	O
for	O
client	O
objects	O
to	O
modify	O
values	O
.	O
</s>
<s>
An	O
upshot	O
of	O
this	O
is	O
that	O
"	O
setters	O
"	O
can	O
,	O
and	O
normally	O
do	O
,	O
implement	O
the	O
invariants	O
for	O
which	O
Eiffel	B-Language
provides	O
syntax	O
.	O
</s>
<s>
While	O
Eiffel	B-Language
does	O
not	O
allow	O
direct	O
access	O
to	O
the	O
features	O
of	O
a	O
class	O
by	O
a	O
client	O
of	O
the	O
class	O
,	O
it	O
does	O
allow	O
for	O
the	O
definition	O
of	O
an	O
"	O
assigner	O
command	O
"	O
,	O
such	O
as	O
:	O
</s>
<s>
thereby	O
breaking	O
the	O
Information	B-Application
Hiding	I-Application
Principle	O
)	O
,	O
the	O
practice	O
is	O
dangerous	O
as	O
it	O
hides	O
or	O
obfuscates	O
the	O
reality	O
of	O
a	O
"	O
setter	O
"	O
being	O
used	O
.	O
</s>
<s>
Unlike	O
other	O
languages	O
,	O
having	O
notions	O
of	O
"	O
public	O
"	O
,	O
"	O
protected	O
"	O
,	O
"	O
private	O
"	O
and	O
so	O
on	O
,	O
Eiffel	B-Language
uses	O
an	O
exporting	O
technology	O
to	O
more	O
precisely	O
control	O
the	O
scoping	O
between	O
client	O
and	O
supplier	O
classes	O
.	O
</s>
<s>
Feature	O
visibility	B-Application
is	O
checked	O
statically	O
at	O
compile-time	O
.	O
</s>
<s>
Finally	O
,	O
scoping	O
can	O
be	O
selectively	O
and	O
precisely	O
controlled	O
to	O
any	O
class	O
in	O
the	O
Eiffel	B-Language
project	O
universe	O
,	O
such	O
as	O
:	O
</s>
<s>
Here	O
,	O
the	O
compiler	B-Language
will	O
allow	O
only	O
the	O
classes	O
listed	O
between	O
the	O
curly	O
braces	O
to	O
access	O
the	O
features	O
within	O
the	O
feature	O
group	O
(	O
e.g.	O
</s>
<s>
Such	O
a	O
program	O
written	O
in	O
Eiffel	B-Language
might	O
be	O
:	O
</s>
<s>
The	O
concept	O
of	O
Design	B-Application
by	I-Application
Contract	I-Application
is	O
central	O
to	O
Eiffel	B-Language
.	O
</s>
<s>
Class	B-Application
Invariant	I-Application
contracts	O
define	O
what	O
assertions	O
must	O
hold	O
true	O
both	O
before	O
and	O
after	O
any	O
feature	O
of	O
a	O
class	O
is	O
accessed	O
(	O
both	O
routines	O
and	O
attributes	O
)	O
.	O
</s>
<s>
The	O
Eiffel	B-Language
compiler	B-Language
is	O
designed	O
to	O
include	O
the	O
feature	O
and	O
class	O
contracts	O
in	O
various	O
levels	O
.	O
</s>
<s>
EiffelStudio	B-Application
,	O
for	O
example	O
,	O
executes	O
all	O
feature	O
and	O
class	O
contracts	O
during	O
execution	O
in	O
the	O
"	O
Workbench	O
mode.	O
"	O
</s>
<s>
When	O
an	O
executable	O
is	O
created	O
,	O
the	O
compiler	B-Language
is	O
instructed	O
by	O
way	O
of	O
the	O
project	O
settings	O
file	O
(	O
e.g.	O
</s>
<s>
Thus	O
,	O
an	O
executable	O
file	O
can	O
be	O
compiled	B-Language
to	O
either	O
include	O
or	O
exclude	O
any	O
level	O
of	O
contract	O
,	O
thereby	O
bringing	O
along	O
continuous	O
levels	O
of	O
unit	O
and	O
integration	O
testing	O
.	O
</s>
<s>
Moreover	O
,	O
contracts	O
can	O
be	O
continually	O
and	O
methodically	O
exercised	O
by	O
way	O
of	O
the	O
Auto-Test	O
feature	O
found	O
in	O
EiffelStudio	B-Application
.	O
</s>
<s>
The	O
Design	B-Application
by	I-Application
Contract	I-Application
mechanisms	O
are	O
tightly	O
integrated	O
with	O
the	O
language	O
and	O
guide	O
redefinition	B-Language
of	O
features	O
in	O
inheritance	B-Language
:	O
</s>
<s>
Routine	O
precondition	O
:	O
The	O
precondition	O
may	O
only	O
be	O
weakened	O
by	O
inheritance	B-Language
;	O
any	O
call	O
that	O
meets	O
the	O
requirements	O
of	O
the	O
ancestor	O
meets	O
those	O
of	O
the	O
descendant	O
.	O
</s>
<s>
Routine	O
postcondition	O
:	O
The	O
postcondition	O
can	O
only	O
be	O
strengthened	O
by	O
inheritance	B-Language
;	O
any	O
result	O
guaranteed	O
by	O
the	O
ancestor	O
is	O
still	O
provided	O
by	O
the	O
descendant	O
.	O
</s>
<s>
Class	B-Application
invariant	I-Application
:	O
Conditions	O
that	O
must	O
hold	O
true	O
after	O
the	O
object	O
's	O
creation	O
and	O
after	O
any	O
call	O
to	O
an	O
exported	O
class	O
routine	O
.	O
</s>
<s>
In	O
fact	O
,	O
void-safe	O
capability	O
could	O
be	O
seen	O
as	O
an	O
extension	O
to	O
the	O
type	O
system	O
,	O
or	O
a	O
step	O
beyond	O
static	O
typing	O
,	O
because	O
the	O
mechanism	O
for	O
ensuring	O
void	B-Application
safety	I-Application
is	O
integrated	O
into	O
the	O
type	O
system	O
.	O
</s>
<s>
The	O
code	O
example	O
above	O
shows	O
how	O
the	O
compiler	B-Language
can	O
statically	O
address	O
the	O
reliability	O
of	O
whether	O
some_attribute	O
will	O
be	O
attached	O
or	O
detached	O
at	O
the	O
point	O
it	O
is	O
used	O
.	O
</s>
<s>
The	O
command-query	O
distinction	O
is	O
important	O
to	O
the	O
Eiffel	B-Language
method	O
.	O
</s>
<s>
Command-Query	B-Application
Separation	I-Application
Principle	I-Application
:	O
Queries	O
must	O
not	O
modify	O
the	O
instance	O
.	O
</s>
<s>
So	O
in	O
good	O
Eiffel	B-Language
style	O
,	O
one	O
does	O
not	O
find	O
"	O
get	O
"	O
functions	O
that	O
change	O
something	O
and	O
return	O
a	O
result	O
;	O
instead	O
there	O
are	O
commands	O
(	O
procedures	O
)	O
to	O
change	O
objects	O
,	O
and	O
queries	O
to	O
obtain	O
information	O
about	O
the	O
object	O
,	O
resulting	O
from	O
preceding	O
changes	O
.	O
</s>
<s>
Eiffel	B-Language
does	O
not	O
allow	O
argument	O
overloading	B-Language
.	O
</s>
<s>
It	O
also	O
simplifies	O
the	O
language	O
mechanism	O
;	O
in	O
particular	O
,	O
this	O
is	O
what	O
makes	O
Eiffel	B-Language
's	O
multiple	B-Application
inheritance	I-Application
mechanism	O
possible	O
.	O
</s>
<s>
A	O
generic	B-Language
class	I-Language
is	O
a	O
class	O
that	O
varies	O
by	O
type	O
(	O
e.g.	O
</s>
<s>
Classes	O
can	O
be	O
generic	B-Language
,	O
to	O
express	O
that	O
they	O
are	O
parameterized	O
by	O
types	O
.	O
</s>
<s>
Generic	B-Language
parameters	O
appear	O
in	O
square	O
brackets	O
:	O
</s>
<s>
G	O
is	O
known	O
as	O
a	O
"	O
formal	O
generic	B-Language
parameter	O
"	O
.	O
</s>
<s>
(	O
Eiffel	B-Language
reserves	O
"	O
argument	O
"	O
for	O
routines	O
,	O
and	O
uses	O
"	O
parameter	O
"	O
only	O
for	O
generic	B-Language
classes	O
.	O
)	O
</s>
<s>
The	O
LIST	O
 [ INTEGER ] 	O
and	O
LIST	O
 [ WORD ] 	O
are	O
"	O
generic	B-Language
derivations	O
"	O
of	O
this	O
class	O
.	O
</s>
<s>
INTEGER	O
and	O
WORD	O
are	O
the	O
"	O
actual	O
generic	B-Language
parameters	O
"	O
in	O
these	O
generic	B-Language
derivations	O
.	O
</s>
<s>
a	O
derivation	O
HASH_TABLE	O
[	O
INTEGER	O
,	O
STRING ]	O
is	O
valid	O
only	O
if	O
STRING	O
inherits	O
from	O
HASHABLE	O
(	O
as	O
it	O
indeed	O
does	O
in	O
typical	O
Eiffel	B-Language
libraries	O
)	O
.	O
</s>
<s>
The	O
class	O
may	O
redefine	O
(	O
override	B-Language
)	O
some	O
or	O
all	O
of	O
the	O
inherited	O
features	O
.	O
</s>
<s>
See	O
for	O
a	O
complete	O
discussion	O
of	O
Eiffel	B-Language
inheritance	B-Language
.	O
</s>
<s>
Non-instantiatable	O
classes	O
are	O
called	O
abstract	O
classes	O
in	O
some	O
other	O
object-oriented	B-Language
programming	I-Language
languages	I-Language
.	O
</s>
<s>
In	O
Eiffel	B-Language
parlance	O
,	O
only	O
an	O
"	O
effective	O
"	O
class	O
can	O
be	O
instantiated	O
(	O
it	O
may	O
be	O
a	O
descendant	O
of	O
a	O
deferred	O
class	O
)	O
.	O
</s>
<s>
Deferred	O
classes	O
play	O
some	O
of	O
the	O
same	O
role	O
as	O
interfaces	B-Application
in	O
languages	O
such	O
as	O
Java	B-Language
,	O
though	O
many	O
object-oriented	B-Language
programming	I-Language
theorists	O
believe	O
interfaces	B-Application
are	O
themselves	O
largely	O
an	O
answer	O
to	O
Java	B-Language
's	O
lack	O
of	O
multiple	B-Application
inheritance	I-Application
(	O
which	O
Eiffel	B-Language
has	O
)	O
.	O
</s>
<s>
This	O
is	O
required	O
in	O
the	O
case	O
of	O
multiple	B-Application
inheritance	I-Application
if	O
there	O
are	O
name	O
clashes	O
between	O
inherited	O
features	O
;	O
without	O
renaming	O
,	O
the	O
resulting	O
class	O
would	O
violate	O
the	O
no-overloading	O
principle	O
noted	O
above	O
and	O
hence	O
would	O
be	O
invalid	O
.	O
</s>
<s>
(	O
In	O
some	O
compilers	B-Language
this	O
is	O
the	O
only	O
form	O
of	O
tuple	O
,	O
as	O
tags	O
were	O
introduced	O
with	O
the	O
ECMA	O
standard	O
.	O
)	O
</s>
<s>
TUPLE	O
[	O
A	O
,	O
B	O
,	O
C ]	O
is	O
that	O
it	O
describes	O
sequences	O
of	O
at	O
least	O
three	O
elements	O
,	O
the	O
first	O
three	O
being	O
of	O
types	O
A	O
,	O
B	O
,	O
C	B-Language
respectively	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
TUPLE	O
[	O
A	O
,	O
B	O
,	O
C ]	O
conforms	O
to	O
(	O
may	O
be	O
assigned	O
to	O
)	O
TUPLE	O
[	O
A	O
,	O
B ]	O
,	O
to	O
TUPLE	O
 [ A ] 	O
and	O
to	O
TUPLE	O
(	O
without	O
parameters	O
)	O
,	O
the	O
topmost	O
tuple	O
type	O
to	O
which	O
all	O
tuple	O
types	O
conform	O
.	O
</s>
<s>
Eiffel	B-Language
's	O
"	O
agent	O
"	O
mechanism	O
wraps	O
operations	O
into	O
objects	O
.	O
</s>
<s>
This	O
mechanism	O
can	O
be	O
used	O
for	O
iteration	O
,	O
event-driven	B-Application
programming	I-Application
,	O
and	O
other	O
contexts	O
in	O
which	O
it	O
is	O
useful	O
to	O
pass	O
operations	O
around	O
the	O
program	O
structure	O
.	O
</s>
<s>
Other	O
programming	O
languages	O
,	O
especially	O
ones	O
that	O
emphasize	O
functional	B-Language
programming	I-Language
,	O
allow	O
a	O
similar	O
pattern	O
using	O
continuations	O
,	O
closures	B-Language
,	O
or	O
generators	O
;	O
Eiffel	B-Language
's	O
agents	O
emphasize	O
the	O
language	O
's	O
object-oriented	B-Language
paradigm	O
,	O
and	O
use	O
a	O
syntax	O
and	O
semantics	O
similar	O
to	O
code	O
blocks	O
in	O
Smalltalk	B-Application
and	O
Ruby	B-Language
.	O
</s>
<s>
The	O
distinction	O
between	O
open	O
and	O
closed	O
operands	O
(	O
operands	O
=	O
arguments	O
+	O
target	O
)	O
corresponds	O
to	O
the	O
distinction	O
between	O
bound	O
and	O
free	O
variables	O
in	O
lambda	B-Language
calculus	I-Language
.	O
</s>
<s>
An	O
agent	O
expression	O
such	O
as	O
action2	O
(	O
?,	O
y	O
)	O
with	O
some	O
operands	O
closed	O
and	O
some	O
open	O
corresponds	O
to	O
a	O
version	O
of	O
the	O
original	O
operation	O
curried	B-Application
on	O
the	O
closed	O
operands	O
.	O
</s>
<s>
Eiffel	B-Language
provides	O
a	O
mechanism	O
to	O
allow	O
conversions	O
between	O
various	O
types	O
.	O
</s>
<s>
The	O
mechanisms	O
coexists	O
with	O
inheritance	B-Language
and	O
complements	O
it	O
.	O
</s>
<s>
as	O
a	O
shortcut	O
for	O
using	O
an	O
explicit	O
object	O
creation	O
with	O
a	B-Language
conversion	I-Language
procedure	O
:	O
</s>
<s>
Exception	B-General_Concept
handling	I-General_Concept
in	O
Eiffel	B-Language
is	O
based	O
on	O
the	O
principles	O
of	O
design	B-Application
by	I-Application
contract	I-Application
.	O
</s>
<s>
In	O
Eiffel	B-Language
,	O
exception	B-General_Concept
handling	I-General_Concept
is	O
not	O
used	O
for	O
control	O
flow	O
or	O
to	O
correct	O
data-input	O
mistakes	O
.	O
</s>
<s>
An	O
Eiffel	B-Language
exception	B-General_Concept
handler	I-General_Concept
is	O
defined	O
using	O
the	O
keyword	O
.	O
</s>
<s>
A	O
concurrency	O
model	O
for	O
Eiffel	B-Language
,	O
based	O
on	O
the	O
concepts	O
of	O
design	B-Application
by	I-Application
contract	I-Application
,	O
is	O
SCOOP	B-Application
,	O
or	O
Simple	O
Concurrent	B-Application
Object-Oriented	I-Application
Programming	I-Application
,	O
not	O
yet	O
part	O
of	O
the	O
official	O
language	O
definition	O
but	O
available	O
in	O
EiffelStudio	B-Application
.	O
</s>
<s>
CAMEO	O
is	O
an	O
(	O
unimplemented	O
)	O
variation	O
of	O
SCOOP	B-Application
for	O
Eiffel	B-Language
.	O
</s>
<s>
Eiffel	B-Language
's	O
view	O
of	O
computation	O
is	O
completely	O
object-oriented	B-Language
in	O
the	O
sense	O
that	O
every	O
operation	O
is	O
relative	O
to	O
an	O
object	O
,	O
the	O
"	O
target	O
"	O
.	O
</s>
<s>
This	O
is	O
particularly	O
useful	O
for	O
container	O
structures	O
such	O
as	O
arrays	O
,	O
hash	B-Algorithm
tables	I-Algorithm
,	O
lists	O
etc	O
.	O
</s>
<s>
"	O
Assigner	O
commands	O
"	O
are	O
a	O
companion	O
mechanism	O
designed	O
in	O
the	O
same	O
spirit	O
of	O
allowing	O
well-established	O
,	O
convenient	O
notation	O
reinterpreted	O
in	O
the	O
framework	O
of	O
object-oriented	B-Language
programming	I-Language
.	O
</s>
<s>
An	O
assignment	O
proper	O
can	O
never	O
be	O
of	O
the	O
form	O
a.x	O
:=	O
v	O
as	O
this	O
violates	O
information	B-Application
hiding	I-Application
;	O
you	O
have	O
to	O
go	O
for	O
a	O
setter	O
command	O
(	O
procedure	O
)	O
.	O
</s>
<s>
Eiffel	B-Language
is	O
not	O
case-sensitive	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
the	O
structure	O
of	O
an	O
Eiffel	B-Language
class	O
is	O
simple	O
:	O
some	O
class-level	O
clauses	O
(	O
inheritance	B-Language
,	O
invariant	O
)	O
and	O
a	O
succession	O
of	O
feature	O
declarations	O
,	O
all	O
at	O
the	O
same	O
level	O
.	O
</s>
<s>
In	O
contrast	O
to	O
most	O
curly	O
bracket	O
programming	O
languages	O
,	O
Eiffel	B-Language
makes	O
a	O
clear	O
distinction	O
between	O
expressions	O
and	O
instructions	O
.	O
</s>
<s>
This	O
is	O
in	O
line	O
with	O
the	O
Command-Query	B-Application
Separation	I-Application
principle	I-Application
of	O
the	O
Eiffel	B-Language
method	O
.	O
</s>
<s>
Much	O
of	O
the	O
documentation	O
of	O
Eiffel	B-Language
uses	O
distinctive	O
style	O
conventions	O
,	O
designed	O
to	O
enforce	O
a	O
consistent	O
look-and-feel	O
.	O
</s>
<s>
Some	O
of	O
these	O
conventions	O
apply	O
to	O
the	O
code	O
format	O
itself	O
,	O
and	O
others	O
to	O
the	O
standard	O
typographic	O
rendering	O
of	O
Eiffel	B-Language
code	O
in	O
formats	O
and	O
publications	O
where	O
these	O
conventions	O
are	O
possible	O
.	O
</s>
<s>
The	O
specification	O
of	O
Eiffel	B-Language
includes	O
guidelines	O
for	O
displaying	O
software	O
texts	O
in	O
typeset	O
formats	O
:	O
keywords	O
in	O
bold	O
,	O
user-defined	O
identifiers	O
and	O
constants	O
are	O
shown	O
in	O
italics	O
,	O
comments	O
,	O
operators	O
,	O
and	O
punctuation	O
marks	O
in	O
Roman	O
,	O
with	O
program	O
text	O
in	O
blue	O
as	O
in	O
the	O
present	O
article	O
to	O
distinguish	O
it	O
from	O
explanatory	O
text	O
.	O
</s>
<s>
program	O
given	O
above	O
would	O
be	O
rendered	O
as	O
below	O
in	O
Eiffel	B-Language
documentation	O
:	O
</s>
<s>
Eiffel	B-Language
is	O
a	O
purely	O
object-oriented	B-Language
language	I-Language
but	O
provides	O
an	O
open	B-Architecture
architecture	I-Architecture
for	O
interfacing	O
with	O
"	O
external	O
"	O
software	O
in	O
any	O
other	O
programming	O
language	O
.	O
</s>
<s>
It	O
is	O
possible	O
for	O
example	O
to	O
program	O
machine	O
-	O
and	O
operating-system	O
level	O
operations	O
in	O
C	B-Language
.	O
Eiffel	B-Language
provides	O
a	O
straightforward	O
interface	O
to	O
C	B-Language
routines	O
,	O
including	O
support	O
for	O
"	O
inline	O
C	B-Language
"	O
(	O
writing	O
the	O
body	O
of	O
an	O
Eiffel	B-Language
routine	O
in	O
C	B-Language
,	O
typically	O
for	O
short	O
machine-level	O
operations	O
)	O
.	O
</s>
<s>
Although	O
there	O
is	O
no	O
direct	O
connection	O
between	O
Eiffel	B-Language
and	O
C	B-Language
,	O
many	O
Eiffel	B-Language
compilers	B-Language
(	O
Visual	B-Application
Eiffel	I-Application
is	O
one	O
exception	O
)	O
output	O
C	B-Language
source	O
code	O
as	O
an	O
intermediate	O
language	O
,	O
to	O
submit	O
to	O
a	O
C	B-Language
compiler	B-Language
,	O
for	O
optimizing	O
and	O
portability	O
.	O
</s>
<s>
As	O
such	O
,	O
they	O
are	O
examples	O
of	O
transcompilers	B-Language
.	O
</s>
<s>
The	O
Eiffel	B-Language
Compiler	B-Language
tecomp	O
can	O
execute	O
Eiffel	B-Language
code	O
directly	O
(	O
like	O
an	O
interpreter	O
)	O
without	O
going	O
via	O
an	O
intermediate	O
C	B-Language
code	O
or	O
emit	O
C	B-Language
code	O
which	O
will	O
be	O
passed	O
to	O
a	O
C	B-Language
compiler	B-Language
in	O
order	O
to	O
obtain	O
optimized	O
native	O
code	O
.	O
</s>
<s>
On	O
.NET	O
,	O
the	O
EiffelStudio	B-Application
compiler	B-Language
directly	O
generates	O
CIL	O
(	O
Common	O
Intermediate	O
Language	O
)	O
code	O
.	O
</s>
<s>
The	O
SmartEiffel	B-Application
compiler	B-Language
can	O
also	O
output	O
Java	B-Language
bytecode	I-Language
.	O
</s>
