<s>
Unlambda	B-Language
is	O
a	O
minimal	O
,	O
"	O
nearly	O
pure	B-Application
"	O
functional	B-Language
programming	I-Language
language	I-Language
invented	O
by	O
David	O
Madore	O
.	O
</s>
<s>
It	O
is	O
based	O
on	O
combinatory	B-Application
logic	I-Application
,	O
an	O
expression	O
system	O
without	O
the	O
lambda	B-Language
operator	I-Language
or	O
free	O
variables	O
.	O
</s>
<s>
These	O
alone	O
make	O
it	O
Turing-complete	O
,	O
but	O
there	O
are	O
also	O
some	O
input/output	B-General_Concept
(	O
I/O	B-General_Concept
)	O
functions	O
to	O
enable	O
interacting	O
with	O
the	O
user	O
,	O
some	O
shortcut	O
functions	O
,	O
and	O
a	O
lazy	O
evaluation	O
function	O
.	O
</s>
<s>
Unlambda	B-Language
is	O
free	B-License
and	I-License
open-source	I-License
software	I-License
distributed	O
under	O
a	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
(	O
GPL	B-License
)	O
2.0	O
or	O
later	O
.	O
</s>
<s>
As	O
an	O
esoteric	O
programming	O
language	O
,	O
Unlambda	B-Language
is	O
meant	O
as	O
a	O
demonstration	O
of	O
very	O
pure	B-Application
functional	I-Application
programming	I-Application
rather	O
than	O
for	O
practical	O
use	O
.	O
</s>
<s>
Data	O
can	O
nevertheless	O
be	O
simulated	O
with	O
appropriate	O
functions	O
as	O
in	O
the	O
lambda	B-Language
calculus	I-Language
.	O
</s>
<s>
Multi-parameter	O
functions	O
can	O
be	O
represented	O
via	O
the	O
method	O
of	O
currying	B-Application
.	O
</s>
<s>
Unlambda	B-Language
is	O
based	O
on	O
the	O
principle	O
of	O
abstraction	B-Application
elimination	I-Application
,	O
or	O
the	O
elimination	O
of	O
all	O
saved	O
variables	O
,	O
including	O
functions	O
.	O
</s>
<s>
As	O
a	O
purely	O
functional	B-Language
language	I-Language
,	O
Unlambda	B-Language
's	O
functions	O
are	O
first-class	O
objects	O
,	O
and	O
are	O
the	O
only	O
such	O
objects	O
.	O
</s>
<s>
Here	O
is	O
an	O
implementation	O
of	O
a	O
hello	O
world	O
program	O
in	O
Unlambda	B-Language
:	O
</s>
<s>
Other	O
important	O
features	O
provided	O
by	O
Unlambda	B-Language
include	O
the	O
k	O
and	O
s	O
functions	O
.	O
</s>
<s>
It	O
is	O
a	O
remarkable	O
fact	O
that	O
s	O
and	O
k	O
are	O
sufficient	O
to	O
perform	O
any	O
calculation	O
,	O
as	O
described	O
in	O
SKI	B-Application
combinator	I-Application
calculus	I-Application
.	O
</s>
<s>
Unlambda	B-Language
's	O
one	O
flow	O
control	O
construct	O
is	O
call	O
with	O
current	O
continuation	O
,	O
denoted	O
c	B-Language
.	O
When	O
an	O
expression	O
of	O
the	O
form	O
`cx	O
is	O
evaluated	O
,	O
a	O
special	O
continuation	O
object	O
is	O
constructed	O
,	O
representing	O
the	O
state	O
of	O
the	O
interpreter	O
at	O
that	O
moment	O
.	O
</s>
<s>
Unlambda	B-Language
's	O
execution	O
semantics	O
are	O
normally	O
eager	O
evaluation	O
,	O
but	O
a	O
lazy	O
evaluation	O
option	O
exists	O
,	O
indicated	O
by	O
the	O
use	O
of	O
the	O
d	O
operator	O
.	O
</s>
<s>
Usually	O
,	O
to	O
evaluate	O
an	O
expression	O
of	O
the	O
form	O
`xy	O
,	O
unlambda	B-Language
first	O
evaluates	O
x	O
,	O
then	O
y	O
,	O
and	O
then	O
applies	O
x	O
to	O
y	O
.	O
</s>
<s>
Unlambda	B-Language
's	O
next	O
built-in	O
operator	O
is	O
v	O
,	O
which	O
ignores	O
its	O
argument	O
and	O
returns	O
v	O
.	O
This	O
feature	O
is	O
not	O
strictly	O
necessary	O
,	O
since	O
v	O
could	O
be	O
implemented	O
as	O
``s`k``s``s`kskk`k``s``s`kskk	O
,	O
but	O
it	O
is	O
supplied	O
as	O
a	O
convenience	O
.	O
</s>
<s>
(	O
This	O
expression	O
above	O
is	O
simply	O
`Yk	O
,	O
where	O
Y	O
denotes	O
a	O
fixed	B-Application
point	I-Application
combinator	I-Application
.	O
)	O
</s>
<s>
More	O
built-ins	O
were	O
introduced	O
in	O
Unlambda	B-Language
version	O
2	O
.	O
</s>
<s>
Input	B-General_Concept
is	O
facilitated	O
by	O
operators	O
@	O
and	O
?	O
u	O
.	O
</s>
<s>
When	O
@	O
is	O
applied	O
to	O
a	O
function	O
x	O
,	O
a	O
character	O
is	O
read	O
from	O
input	B-General_Concept
,	O
and	O
stored	O
as	O
the	O
"	O
current	O
character	O
"	O
;	O
then	O
x	O
is	O
applied	O
to	O
i	O
.	O
</s>
<s>
However	O
,	O
if	O
no	O
more	O
characters	O
were	O
available	O
on	O
input	B-General_Concept
,	O
the	O
current	O
character	O
is	O
left	O
undefined	O
,	O
and	O
x	O
is	O
applied	O
to	O
v	O
instead	O
.	O
</s>
