<s>
Algebraic	B-Application
Logic	I-Application
Functional	I-Application
programming	I-Application
language	I-Application
,	O
also	O
known	O
as	O
ALF	O
,	O
is	O
a	O
programming	O
language	O
which	O
combines	O
functional	B-Language
and	O
logic	B-Language
programming	I-Language
techniques	O
.	O
</s>
<s>
Its	O
foundation	O
is	O
Horn	B-Application
clause	I-Application
logic	B-Language
with	O
equality	O
which	O
consists	O
of	O
predicates	O
and	O
Horn	B-Application
clauses	I-Application
for	O
logic	B-Language
programming	I-Language
,	O
and	O
functions	O
and	O
equations	O
for	O
functional	B-Language
programming	I-Language
.	O
</s>
<s>
ALF	O
was	O
designed	O
to	O
be	O
genuine	O
integration	O
of	O
both	O
programming	O
paradigms	O
,	O
and	O
thus	O
any	O
functional	B-Language
expression	O
can	O
be	O
used	O
in	O
a	O
goal	O
literal	O
and	O
arbitrary	O
predicates	O
can	O
occur	O
in	O
conditions	O
of	O
equations	O
.	O
</s>
<s>
ALF	O
's	O
operational	O
semantics	O
is	O
based	O
on	O
the	O
resolution	O
rule	O
to	O
solve	O
literals	O
and	O
narrowing	O
to	O
evaluate	O
functional	B-Language
expressions	O
.	O
</s>
<s>
Rewriting	O
and	O
rejection	O
are	O
supposed	O
to	O
result	O
in	O
a	O
large	O
reduction	O
of	O
the	O
search	O
tree	O
and	O
produce	O
an	O
operational	O
semantics	O
that	O
is	O
more	O
efficient	O
than	O
Prolog	B-Language
's	I-Language
resolution	O
strategy	O
.	O
</s>
<s>
Similarly	O
to	O
Prolog	B-Language
,	O
ALF	O
uses	O
a	O
backtracking	O
strategy	O
corresponding	O
to	O
a	O
depth-first	B-Algorithm
search	I-Algorithm
in	O
the	O
derivation	O
tree	O
.	O
</s>
<s>
The	O
abstract	O
machine	O
is	O
based	O
on	O
the	O
Warren	B-Application
Abstract	I-Application
Machine	I-Application
(	O
WAM	O
)	O
with	O
several	O
extensions	O
to	O
implement	O
narrowing	O
and	O
rewriting	O
.	O
</s>
<s>
In	O
the	O
current	O
ALF	O
implementation	O
programs	O
of	O
this	O
abstract	O
machine	O
are	O
executed	O
by	O
an	O
emulator	O
written	O
in	O
C	B-Language
.	O
</s>
<s>
In	O
the	O
Carnegie	O
Mellon	O
University	O
Artificial	B-Application
Intelligence	I-Application
Repository	O
,	O
ALF	O
is	O
included	O
as	O
an	O
AI	B-Application
programming	O
language	O
,	O
in	O
particular	O
as	O
a	O
functional/logic	O
programming	O
language	O
Prolog	B-Language
implementation	O
.	O
</s>
<s>
The	O
ALF	O
System	O
runs	O
under	O
Unix	B-Application
and	O
is	O
available	O
under	O
a	O
custom	O
proprietary	B-Application
software	I-Application
license	I-Application
that	O
grants	O
the	O
right	O
to	O
use	O
for	O
"	O
evaluation	O
,	O
research	O
and	O
teaching	O
purposes	O
"	O
but	O
not	O
commercial	O
or	O
military	O
use	O
.	O
</s>
