<s>
CLU	B-Language
is	O
a	O
programming	O
language	O
created	O
at	O
the	O
Massachusetts	O
Institute	O
of	O
Technology	O
(	O
MIT	O
)	O
by	O
Barbara	O
Liskov	O
and	O
her	O
students	O
starting	O
in	O
1973	O
.	O
</s>
<s>
While	O
it	O
did	O
not	O
find	O
extensive	O
use	O
,	O
it	O
introduced	O
many	O
features	O
that	O
are	O
used	O
widely	O
now	O
,	O
and	O
is	O
seen	O
as	O
a	O
step	O
in	O
the	O
development	O
of	O
object-oriented	B-Language
programming	I-Language
(	O
OOP	O
)	O
.	O
</s>
<s>
Key	O
contributions	O
include	O
abstract	O
data	O
types	O
,	O
call-by-sharing	O
,	O
iterators	O
,	O
multiple	O
return	O
values	O
(	O
a	O
form	O
of	O
parallel	O
assignment	O
)	O
,	O
type-safe	O
parameterized	B-Language
types	I-Language
,	O
and	O
type-safe	O
variant	B-Language
types	I-Language
.	O
</s>
<s>
It	O
is	O
also	O
notable	O
for	O
its	O
use	O
of	O
classes	O
with	O
constructors	O
and	O
methods	O
,	O
but	O
without	O
inheritance	B-Language
.	O
</s>
<s>
The	O
syntax	B-Application
of	O
CLU	B-Language
was	O
based	O
on	O
ALGOL	B-Language
,	O
then	O
the	O
starting	O
point	O
for	O
most	O
new	O
language	O
designs	O
.	O
</s>
<s>
The	O
key	O
addition	O
was	O
the	O
concept	O
of	O
a	O
cluster	O
,	O
CLU	B-Language
's	O
type	O
extension	O
system	O
and	O
the	O
root	O
of	O
the	O
language	O
's	O
name	O
(	O
CLUster	O
)	O
.	O
</s>
<s>
For	O
instance	O
,	O
here	O
is	O
the	O
CLU	B-Language
syntax	B-Application
for	O
a	O
cluster	O
that	O
implements	O
complex	O
numbers	O
:	O
</s>
<s>
CLU	B-Language
does	O
not	O
otherwise	O
perform	O
implicit	O
type	O
conversions	O
.	O
</s>
<s>
Another	O
key	O
feature	O
of	O
the	O
CLU	B-Language
type	O
system	O
are	O
iterators	O
,	O
which	O
return	O
objects	O
from	O
a	O
collection	O
serially	O
,	O
one	O
after	O
another	O
.	O
</s>
<s>
Iterators	O
offer	O
an	O
identical	O
application	B-Application
programming	I-Application
interface	I-Application
(	O
API	B-Application
)	O
no	O
matter	O
what	O
data	O
they	O
are	O
being	O
used	O
with	O
.	O
</s>
<s>
A	O
distinctive	O
feature	O
of	O
CLU	B-Language
iterators	O
is	O
that	O
they	O
are	O
implemented	O
as	O
coroutines	O
,	O
with	O
each	O
value	O
being	O
provided	O
to	O
the	O
caller	O
via	O
a	O
yield	O
statement	O
.	O
</s>
<s>
Iterators	O
like	O
those	O
in	O
CLU	B-Language
are	O
now	O
a	O
common	O
feature	O
of	O
many	O
modern	O
languages	O
,	O
such	O
as	O
C#	B-Application
,	O
Ruby	B-Language
,	O
and	O
Python	B-Language
,	O
though	O
recently	O
they	O
are	O
often	O
referred	O
to	O
as	O
generators	O
.	O
</s>
<s>
CLU	B-Language
also	O
includes	O
exception	B-General_Concept
handling	I-General_Concept
,	O
based	O
on	O
various	O
attempts	O
in	O
other	O
languages	O
;	O
exceptions	O
are	O
raised	O
using	O
signal	O
and	O
handled	O
with	O
except	O
.	O
</s>
<s>
Unlike	O
most	O
other	O
languages	O
with	O
exception	B-General_Concept
handling	I-General_Concept
,	O
exceptions	O
are	O
not	O
implicitly	O
resignaled	O
up	O
the	O
calling	O
chain	O
.	O
</s>
<s>
Also	O
unlike	O
most	O
other	O
languages	O
that	O
provide	O
exception	B-General_Concept
handling	I-General_Concept
,	O
exceptions	O
in	O
CLU	B-Language
are	O
considered	O
part	O
of	O
ordinary	O
execution	O
flow	O
and	O
are	O
considered	O
a	O
"	O
normal	O
"	O
and	O
efficient	O
type-safe	O
way	O
to	O
break	O
out	O
of	O
loops	O
or	O
to	O
return	O
from	O
functions	O
;	O
this	O
allows	O
for	O
direct	O
assignment	O
of	O
return	O
values	O
"	O
except	O
when	O
"	O
other	O
conditions	O
apply	O
.	O
</s>
<s>
CLU	B-Language
is	O
often	O
credited	O
as	O
being	O
the	O
first	O
language	O
with	O
type-safe	O
variant	B-Language
types	I-Language
,	O
called	O
oneofs	O
,	O
before	O
the	O
language	O
ML	B-Language
had	O
them	O
.	O
</s>
<s>
A	O
final	O
distinctive	O
feature	O
in	O
CLU	B-Language
is	O
parallel	O
assignment	O
(	O
multiple	O
assignment	O
)	O
,	O
where	O
more	O
than	O
one	O
variable	O
can	O
appear	O
on	O
the	O
left	O
hand	O
side	O
of	O
an	O
assignment	O
operator	O
.	O
</s>
<s>
Parallel	O
assignment	O
(	O
though	O
not	O
multiple	O
return	O
values	O
)	O
predates	O
CLU	B-Language
,	O
appearing	O
in	O
CPL	B-Language
(	O
1963	O
)	O
,	O
named	O
simultaneous	O
assignment	O
,	O
but	O
CLU	B-Language
popularized	O
it	O
and	O
is	O
often	O
credited	O
as	O
the	O
direct	O
influence	O
leading	O
to	O
parallel	O
assignment	O
in	O
later	O
languages	O
.	O
</s>
<s>
All	O
objects	O
in	O
a	O
CLU	B-Language
program	O
live	O
in	O
the	O
heap	O
,	O
and	O
memory	O
management	O
is	O
automatic	O
.	O
</s>
<s>
CLU	B-Language
supports	O
type-parameterized	B-Language
user-defined	O
data	O
abstractions	O
.	O
</s>
<s>
It	O
was	O
the	O
first	O
language	O
to	O
offer	O
type-safe	O
bounded	O
parameterized	B-Language
types	I-Language
,	O
using	O
where	O
clauses	O
to	O
express	O
constraints	O
on	O
actual	O
type	O
arguments	O
.	O
</s>
<s>
Unlike	O
in	O
languages	O
with	O
template-based	O
generics	O
,	O
a	O
use	O
of	O
such	O
a	O
data	O
abstraction	O
can	O
be	O
type-checked	O
without	O
access	O
to	O
the	O
implementation	O
of	O
the	O
abstraction	O
.	O
</s>
<s>
CLU	B-Language
and	O
Ada	B-Language
were	O
major	O
inspirations	O
for	O
C++	B-Application
templates	I-Application
.	O
</s>
<s>
CLU	B-Language
's	O
exception	B-General_Concept
handling	I-General_Concept
mechanisms	O
influenced	O
later	O
languages	O
like	O
C++	B-Language
and	O
Java	B-Language
.	O
</s>
<s>
Sather	B-Application
,	O
Python	B-Language
,	O
and	O
C#	B-Application
include	O
iterators	O
,	O
which	O
first	O
appeared	O
in	O
CLU	B-Language
.	O
</s>
<s>
Perl	B-Language
and	O
Lua	B-Language
took	O
multiple	O
assignment	O
and	O
multiple	O
returns	O
from	O
function	O
calls	O
from	O
CLU	B-Language
.	O
</s>
<s>
Python	B-Language
and	O
Ruby	B-Language
borrowed	O
call	O
by	O
sharing	O
,	O
the	O
yield	O
statement	O
,	O
and	O
multiple	O
assignment	O
.	O
</s>
