2012-09-05 ICLP-2012 1

Jan WielemakerVU UniversityAmsterdam

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Contents

Viewpoint A brief history of SWI-Prolog The Prolog landscape The place of Prolog in IT infrastructure The Prolog software community

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Viewpoint

Logic programming Programming languages Prolog

Prolog implementations Prolog usage Prolog community (user and developers)

Used TPLP special issue on Prolog implementations

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A brief history of SWI-Prolog

1983 D. L. Bowen, L. M. Byrd, and WF. Clocksin. “A portable Prolog compiler”

1986 I wrote a virtual machine based on this paper

1987 Use in European KADS project, replacing Quintus. Distributed using FTP

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Success in KADS

Bi-directional Prolog ↔ C interface allowed for better integration to PCE (graphics)

Quintus compatibility on `as needed' basis to support the project

Fast compiler, lazy loading (autoload) Fast saved-state (based on `undump') and

reload of modified code using ?- make. Command-line editing and completion

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Yet another Prolog(not to br confused YAP)

Satisfied our needs We had full control … but, nothing unique that was not added

to commercial systems within weeks

→ Released FTP Academic `non-commercial use' No need to fill out forms

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Educational

Free of hassle (download, compile, use) Supports popular academic Unix machines Resource friendly Compliant with primitives in textbooks

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The Carrot: quick release model

Input Contributions (mostly portability) Bug reports Functionality requests

Output Fast response Updated source on FTP, ranging from

hours to a week

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The middle ages (1988-2001)

New libraries, bugfixes, etc. ISO part-I Port to Windows 95/NT3.5 Quality (CVS, test suite) But … we missed lots of Prolog research

(parallel, CLP, ...)

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2001: License

GNU libreadline conflicts with non-commercial-use license

FSF: remove libreadline or change SWI-Prolog license

→ SWI-Prolog moved to LGPL Started http://www.swi-prolog.org

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2003: CLP

After Mumbai ICLP, together with Tom Schrijvers and Bart Demoen

Copy of `dynamic attributed variables' as proposed by Demoen and implemented for hProlog

Leuven CHR, coroutining Markus Triska started pure Prolog clp(fd)

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2003.. Web support

Multi-threading Unicode Atom garbage collection SGML/XML HTTP Server and client libraries RDF libraries and store

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Commercial involvement

Contributions JPL, the Java interface Quality checks and bug fixes

Sponsored development Unbounded integers and rationals (GMP) SSL interface Memory management PlDoc, PlUnit IF/Prolog portability layer

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Success

Right time Early hassle-free Prolog for education Early port to Windows (students)

Model Quick release cycle: 2 weeks for

development, 6 months for stable version Technical

Interface, do-what-I-mean, robustness, interfaces

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Failure

Trying to do `everything' Packages often rely on undocumented

SWI-specific extensions, making it hard to reuse them

Improving Development APIs (share with PDT) Modularized I/O and OS interface (YAP) Modularize (GIT) repository

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Part-II

The Prolog landscape

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Is Prolog in BAD shape?

Ohloh (http://www.ohloh.net/)Language Lines of code in projects

Prolog 652,576

Erlang 20,217,024

Haskell 6,636,372

Lisp 19,657,199

Compilers

Puzzles

Applications

Reasoning

Extensions

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Is Prolog in BAD shape?

Discontinued in many Dutch universities or moved to a `language overview course'

Hard to convince our (PhD) students to use it, even for RDF because

Perceived as `hard to learn' No modern (IDE) tools No library for X, Y and Z (you name it) ``An irrelevant language from the past'' ``I always programmed in (often) Java''

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Some often heard reasons

(Early) fragmentation of the language No programmers/No jobs Logic programming/Prolog is unsuitable as

a general purpose language Failed standardization

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Fragmentation?

Prolog Lisp Scheme Haskell C0

10

20

30

40

50

60

70

#Implementations (source: OpenDirectory)

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No programmers?

Language vWorks expertise StackOverflow questions

Java 18,700 286.290

Prolog 453 1845

Lisp 545 2068

Erlang 46 2587

Haskell 115 7854

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Books

19851986198719881989199019911992199319941995199619971998199920002001200220032004200520062007200820092010201120120

1

2

3

4

5

6Source: Amazon (UK)

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The Gartner Hype Cycle Explained

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Part III

The place of Prolog in IT infrastructure

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General Purpose vs Logic server

`General purpose' Missing language features (e.g.,

destructive assignment) Needs graphics, Web, DB, etc., interfaces

`Logic server' Needs language interfaces Suffers from object/relational impedance

mismatchhttp://en.wikipedia.org/wiki/Object-relational_impedance_mismatch

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Logic server

Native (Amzi!) JVM based (Jinni, Minerva, Jekejeke, …)

Successor BinProlog → Jinni IF/Prolog → Minerva

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Embedding is popular SWI-Prolog has interfaces for C (native),

C++ (native and contributed), Java, C#, Python, Perl (contributed)

113 projects on GitHUB combine Prolog with:

Perl Java

Ruby C

Haskell Scala

Erlang Python

C# Common Lisp

Clojure VimL

PHP C++

JavaScript Objective-C

OCaml F#

Emacs Lisp Shell

Scheme Racket

Lua Groovy

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SWI-Prolog: `glue'

Prolog is good at `glue' due to Dynamic nature (incl. typing) Flexible dynamically typed data

representation makes it simple to represent data from external systems

Natural mapping of tabular data Interactive development environment Rules, DSLs, ...

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Some realized `glue' (for SWI)

Databases (ODBC) RDF XML/SGML/HTML R (statistical computing) HTTP Server library Spatial reasoning (geos library) OpenGL, OSC, MIDI, Gtk, Matlab, ...

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Prolog as primary language(SecuritEase)

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Conclusion

`Logic server' seems a more natural and easier to sell first option

Provided with well designed `glue', Prolog is also attractive as primary application language, including scripting

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Part IV

The Prolog software community

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Comparing modelsImplementation Maintenance Progress Shared

APIStable

API

Independent development

duplicate duplicate ++ -- -

Shared design

duplicate duplicate +++ na. na.

Shared specification

duplicate duplicate* -- +++ +++

Forked specification

duplicate duplicate +++ ++ +

Forked code single duplicate ++++ ++ +

Shared code single shared ++ ++++ ++

* Stability of specification makes duplicates easier to handle

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Forking (Unix wars)

Source: Wikipedia

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Distributions (Linux)

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Summary

Shared standards are good but slow. Fits poorly in a culture of innovation.

Shared code leads to Reuse (stand on the shoulders of giants) Fast innovation

Sharing needs a common ground

And, where do we stand?

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Independent Development

Various module systems Predicate based ↔ name based

Operating system access File system, networking

Interfaces DB, Graphics, C, Java, C#, ...

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Shared design

WAM Garbage collectors Tabled evaluation Attributed variables

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Shared specification

ISO standard

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Forked specification

Quintus module system (SICStus, Ciao, YAP, SWI)

hProlog Dynamic attributed vars (SWI, YAP, XSB)

SWI Thread API (YAP, XSB)

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Forked code

DEC10 library (forked many times) clp(q,r) Draxler's Prolog → SQL compiler SWI I/O and OS access (YAP, close to shared) SWI XML/SGML (XSB) SWI PlUnit (SICStus) Parts of XSB tabling (YAP)?

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Shared Prolog components

Logtalk (many) CHR (hProlog, SWI, XSB, YAP, B-Prolog, SICStus, Ciao)

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Requirements for sharing

Common ground for Core Prolog (++) Prolog extensions (attvar,tabling) (+) Structuring (modules, objects) (+/-) Interface to non-Prolog resources (–)http://www.cs.unipr.it/~bagnara/Papers/Prolog-FLI-survey

Always remaining issues → need work-around

Conditional compilation (++) Macro expansion (+)

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Syst./Charac. B-Prolog Bin Ciao Eclipse GNU SICStus SWI XSB YAP

Glue code m m b m b b m(b) b m

Int. style f l f f f p p p f

Term constr. t b b b b b b/t t b

Non-det. n n s n y n y n y

Exceptions n n y n y y y n n

C to Prolog r r r y r r r y r

(De)Init n n i n n b b n n

Compilation m m a m m e m a m

Linking s s d b s b b s d

Comparision of FLI

http://www.cs.unipr.it/~bagnara/Papers/Prolog-FLI-survey

m: manual, f: functional, p: procedural, l: low level, s: static, d: dynamic, b: both

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Progress (Foreign)

FLI emulations SICStus ↔ SWI (Alpino, XPCE) YAP ← SWI (SWI packages) YAP ← Bprolog (PRISM)

Reuse SWI I/O and OS builtins in YAP In part based on FLI emulation Also emulated low-level SWI FLI

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Prolog Commons

Goal: arrive at a shared Prolog library Minimal shared module support Shared document and annotation model

Ciao effort Website, GIT repo

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YAP/SWI`dialect' framework

:- expects_dialect(X) says Text is written for X Loads dialect(X) (= library(dialect/X))

Defines built-ins of X not in target Rewrites conflicting built-ins using macros Rewrites module API using macros Pushes X-compatible library

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Dialects implemented

YAP BProlog, Commons, hProlog, SWI-Prolog

SWI YAP, BIM, hProlog, Commons, Ciao,

IF/Prolog

`As-needed' basis

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Dialect applied (SWI)

Alpino (now maintained as portable code) 500Klines, C,C++,Tcl/Tk SICStus → SWI

Prosyn (ported) 1Mlines, C,C++ (Windows MFC) IF/Prolog → SWI Replaced MFC with web-GUI, other C

with pure Prolog code

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Dialect Applied (YAP)

Many of SWI's extension packages PRISM

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Commons ↔ Dialect

Shared spec/code Slow to start Must be useable

Use each others code

Immediate result `As needed'

I. Use commons as a `dialect'

II. Slowly, a comprehensive commons can act as shared library for many projects

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Distribution: Pack

Simple directory with File pack.pl containing meta-data Sub-dir prolog that is added to the library Optional configure[.in], makefile[.in] for

foreign parts http://www.swi-prolog.org/pack/list

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Conclusions

Prolog was hyped and its community was fragmented. There are signs indicating Prolog reached the enlightenment phase:

New books get published again Compatibility and portable applications

are becoming more common Stability and compiling improved much

(Paulo Moura, CICLOPS) Sharing resources (libraries, applications,

tutorials, books, …) is important

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Next steps (also open.pl)

Organize new `commons' meetings to Bootstrap the common library/dialect Establish a common Prolog ↔ C binding Establish macro expansion A package distribution format

Implement the dialect emulation Share!