Prolog Business Cases
To establish the use of Prolog in an organization, management must
be made aware of the immense benefits it has from
a business perspective, such as:
In any organization, there are countless possible applications
for Prolog. In the following, we consider some of them.
- Prolog is ISO standardized. This is an extremely
important asset from a legal perspective, since it means that
Prolog vendors can be held accountable when
violating the standard.
- Prolog cuts costs: Since Prolog programs are
so general, you typically need less code,
which you can type in less time.
- Prolog has a proven track record in many very
serious applications. See for example the SICStus
- Prolog is ideally suited for rule based systems
that express your business rules.
Prolog as a database
One of the most basic use cases of Prolog is to rely on its
ability to act as a database.
By this, we mean applications that simply store data
and make it available for querying.
Prolog is very well suited for such applications, since you can
dynamically add and remove data, and rely on
automatic indexing to obtain good performance in many
situations of practical relevance. Also, you can easily edit and
process Prolog programs by various means, which makes
exporting, importing and processing the accumulated knowledge in
general very convenient. In addition, formulating Prolog queries
is often more natural and easier than formulating
SQL queries. For such reasons, Prolog is often a good
alternative for other database systems.
There is a syntactic subset of Prolog that is
and it is especially suitable for such use cases. You can
also use the Constraint Query
to seamlessly access SQL databases from Prolog.
Analyzing events and anomalies
A logfile records events.
For example, you can configure a
Prolog HTTPS sever to store
information about requests it processes.
When you locally test such a server by fetching the
page /prolog, it may write an entry
similar to the following in its logfile:
/*Sun Apr 9 23:01:19 2017*/
if_modified_since('Wed, 05 Apr 2017 23:24:08 GMT')]).
This entry describes the request of the client.
After processing the request, the server may write an additional
entry that describes how the request was processed. For example,
it may add:
completed(45, 0.0009790000000000076, 0, 304, not_modified).
The key point is that you can configure the server so that the
logfile is a series of Prolog facts. Hence, the whole
logfile can be easily analyzed using Prolog!
A single invocation of Prolog suffices to load the
entire file, and at the same time makes it ready for
posting arbitrary queries over the accumulated data.
For example, in the case above, we can ask: Are there any
client requests that were not served? In Prolog,
?- request(R, _, _), \+ completed(R, _, _, _, _).
In our case, this means that the server at least handled
each request in some way, and did not silently
Of course, not all logfiles can be parsed directly
with Prolog. However, the more you use Prolog syntax
within your organization, the more you will benefit from being
able to post queries over such data. In addition, you can
typically easily convert any logfile format to valid
Prolog facts, and then apply the same reasoning.
In every organization, there is a constant need for
excellent reports. Prolog is ideally suited
for generating such reports from available data.
For example, imagine you are responsible for reporting the number
of monthly visitors of your company's homepage.
Further, for historic reasons, your team of technicians sends
you the average number of
visitors per day, at the end of each month. That
is, the data you have available is stored in the following
convenient form, using Prolog facts, indicating a nice growth
of daily visitors to your site:
year_month_daily_visitors(2017, jan, 30018).
year_month_daily_visitors(2017, feb, 32913).
year_month_daily_visitors(2017, mar, 35871).
However, your management wants you to report
the total number of visitors for each month, not the
Using Prolog rules, it is straight-forward
to convert your data to the data that
your management wants to see. For example,
using integer arithmetic, we can readily
relate the daily average to the monthly total:
year_month_total(Year, Month, Total) :-
Total #= Days*Daily,
year_month_days(Year, Month, Days),
year_month_daily_visitors(Year, Month, Daily).
This only requires the definition of year_month_days/3,
with clauses that may look as follows:
year_month_days(2017, jan, 31).
year_month_days(2017, feb, 28).
year_month_days(2017, mar, 31).
This suffices to produce the answers we need, for example:
?- year_month_total(Year, Month, Total).
Year = 2017,
Month = jan,
Total = 930558 ;
Year = 2017,
Month = feb,
Total = 921564 ;
Year = 2017,
Month = mar,
Total = 1112001.
Using format/2, you can turn this into a more readable report:
?- format("year month visitors~n", ),
year_month_total(Year, Month, Total),
format("~w ~w ~t~D~20|~n", [Year,Month,Total]),
year month visitors
2017 jan 930,558
2017 feb 921,564
2017 mar 1,112,001
Notice the use of forced backtracking
A few weeks later, management needs a different report: They now
want neither the daily average, nor the total number, but rather
the cumulative sum of visitors for the current
year, up to and including the current month.
Using Prolog, it is straight-forward to generate this report by
simply formulating the required rules, while retaining
the original data exactly as it was. For example:
cumulative(Pred, Until, C) :-
cumulative_(Ms, Pred, Until, 0, C).
cumulative_(, _, _, C, C).
cumulative_([M|Ms], Pred, Until, C0, C) :-
C1 #= C0 + Current,
call(Pred, M, Current),
cumulative_(Ms, Pred, Until, C1, C)).
This uses the
and if_/3 to
describe the cumulative sum in a very general and
flexible way. For example:
?- cumulative(year_month_total(2017), M, C).
M = jan,
C = 930558 ;
M = feb,
C = 1852122 ;
M = mar,
C = 2964123 ;
As a special case of this query, we can of course also inquire any
?- cumulative(year_month_total(2017), feb, C).
C = 1852122 ;
Note the massive advantages of such a way to reason about
your data. For example, it suffices to send
around Prolog facts, which you can easily do via
plain text. Augmenting these facts is also very easy: It
suffices to add one fact per month to keep the above
Suppose you are successfully applying Prolog within your company
to solve difficult problems. Would you publicly announce the fact
that you are using Prolog, so that your competitors can benefit
from the same technology? My personal guess is: No,
Sometimes, when you introduce Prolog in an organization, people
will dismiss the language because they have never heard of anyone
who uses it.
third of all airline tickets is handled by systems that
run SICStus Prolog. NASA uses SICStus Prolog for a
voice-controlled system onboard the International
Space Station. Windows NT used an embedded Prolog
for network configuration.
broking system is written in Prolog and CHR.
A commercial Prolog system easily costs thousands of
dollars. This is not software you "just buy" as an
individual. A professional Prolog system is typically bought by
companies who simply need the power of Prolog to solve
their tasks. They are unlikely to advertise their internal
technologies. Still, a few of them do. For example, the
Virtual Machine Specification contains a lot of
Prolog code, and
uses Prolog for natural language processing.
Sometimes, instead of trying to introduce Prolog in an
organization, it is more efficient to start working for one where
it is already being used!
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