<s>
The	O
ECL	B-Language
programming	I-Language
language	I-Language
and	O
system	O
were	O
an	O
extensible	B-Application
high-level	O
programming	O
language	O
and	O
development	B-Application
environment	I-Application
developed	O
at	O
Harvard	O
University	O
in	O
the	O
1970s	O
.	O
</s>
<s>
The	O
name	O
'	O
ECL	B-Language
 '	O
stood	O
for	O
'	O
Extensible	B-Application
Computer	O
Language	O
 '	O
or	O
'	O
EClectic	O
Language	O
 '	O
.	O
</s>
<s>
Some	O
publications	O
used	O
the	O
name	O
'	O
ECL	B-Language
 '	O
for	O
the	O
system	O
as	O
a	O
whole	O
and	O
EL/1	B-Language
(	O
Extensible	B-Application
Language	I-Application
)	O
for	O
the	O
language	O
.	O
</s>
<s>
ECL	B-Language
was	O
an	O
interactive	O
system	O
where	O
programs	O
were	O
represented	O
within	O
the	O
system	O
;	O
there	O
was	O
a	O
compatible	O
compiler	B-Language
and	O
interpreter	B-Application
.	O
</s>
<s>
It	O
had	O
an	O
ALGOL-like	B-Language
syntax	O
and	O
an	O
extensible	B-Application
data	O
type	O
system	O
,	O
with	O
data	O
types	O
as	O
first-class	O
citizens	O
.	O
</s>
<s>
ECL	B-Language
was	O
primarily	O
used	O
for	O
research	O
and	O
teaching	O
in	O
programming	O
language	O
design	O
,	O
programming	O
methodology	O
(	O
in	O
particular	O
programming	O
by	O
transformational	B-Application
refinement	I-Application
)	O
,	O
and	O
programming	B-Application
environments	I-Application
at	O
Harvard	O
,	O
though	O
it	O
was	O
said	O
to	O
be	O
used	O
at	O
some	O
government	O
agencies	O
as	O
well	O
.	O
</s>
<s>
It	O
was	O
first	O
implemented	O
on	O
the	O
PDP-10	B-Device
,	O
with	O
a	O
later	O
(	O
interpreted-only	O
)	O
implementation	O
on	O
the	O
PDP-11	B-Device
written	O
in	O
BLISS-11	O
and	O
cross-compiled	O
on	O
the	O
PDP-10	B-Device
.	O
</s>
<s>
An	O
ECL	B-Language
procedure	O
for	O
computing	O
the	O
greatest	O
common	O
divisor	O
of	O
two	O
integers	O
according	O
to	O
the	O
Euclidean	O
algorithm	O
could	O
be	O
defined	O
as	O
follows	O
:	O
</s>
<s>
(	O
Data	O
types	O
are	O
called	O
modes	O
in	O
ECL	B-Language
.	O
)	O
</s>
<s>
The	O
computational	O
components	O
of	O
an	O
ECL	B-Language
program	O
are	O
called	O
forms	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
bind-class	O
BYVAL	O
,	O
ECL	B-Language
has	O
bind-classes	O
SHARED	O
,	O
LIKE	O
,	O
UNEVAL	O
,	O
and	O
LISTED	O
.	O
</s>
<s>
Bind-class	O
UNEVAL	O
specifies	O
that	O
an	O
abstract	B-Data_Structure
syntax	I-Data_Structure
tree	I-Data_Structure
for	O
the	O
actual	O
parameter	O
is	O
to	O
be	O
passed	O
to	O
the	O
formal	O
parameter	O
;	O
this	O
provides	O
extraordinary	O
flexibility	O
for	O
programmers	O
to	O
invent	O
their	O
own	O
notations	O
,	O
with	O
their	O
own	O
evaluation	O
semantics	O
,	O
for	O
certain	O
procedure	O
parameters	O
.	O
</s>
<s>
Bind-class	O
LISTED	O
is	O
similar	O
to	O
UNEVAL	O
,	O
but	O
provides	O
a	O
capability	O
similar	O
to	O
that	O
of	O
varargs	B-Language
in	O
C	B-Language
:	O
the	O
LISTED	O
bind-class	O
can	O
only	O
appear	O
in	O
the	O
last	O
formal	O
parameter	O
of	O
the	O
procedure	O
,	O
and	O
that	O
formal	O
parameter	O
is	O
bound	O
to	O
a	O
list	O
of	O
abstract	B-Data_Structure
syntax	I-Data_Structure
tree	I-Data_Structure
representations	I-Data_Structure
,	O
one	O
for	O
each	O
remaining	O
actual	O
parameter	O
.	O
</s>
<s>
ECL	B-Language
has	O
an	O
EVAL	O
built-in	O
function	O
for	O
evaluating	O
an	O
abstract	B-Data_Structure
syntax	I-Data_Structure
tree	I-Data_Structure
;	O
alternatively	O
,	O
there	O
are	O
functions	O
by	O
which	O
programmers	O
can	O
explore	O
the	O
nodes	O
of	O
the	O
abstract	B-Data_Structure
syntax	I-Data_Structure
tree	I-Data_Structure
and	O
process	O
them	O
according	O
to	O
their	O
own	O
logic	O
.	O
</s>
