1.3 Should I be using SWI-Prolog?

There are a number of reasons why it might be better to choose a commercial, or another free, Prolog system:

• SWI-Prolog comes with no warranties
  Although the developers or the community often provide a work-around or a fix for a bug, there is no place you can go to for guaranteed support. However, the full source archive is available and can be used to compile and debug SWI-Prolog using free tools on all major platforms. Users requiring more support should ensure access to knowledgeable developers.

• Performance is your first concern
  Various free and commercial systems have better performance. But,‘standard' Prolog benchmarks disregard many factors that are often critical to the performance of large applications. SWI-Prolog is not good at fast calling of simple predicates, but it is fast with dynamic code, meta-calling and predicates that contain large numbers of clauses or require more advanced clauses indexing. Many of SWI-Prolog's built-in predicates are written in C and have excellent performance.

On the other hand, SWI-Prolog offers some facilities that are widely appreciated by users:

• Comprehensive support of Prolog extensions
  Many modern Prolog implementations extend the standard SLD resolution mechanism with which Prolog started and that is described in the ISO standard. SWI-Prolog offers most popular extensions.

  Attributed variables provide Constraint Logic Programming and delayed execution based on instantiation (coroutining). Tabling or SGL resolution provides characteristics normally associated with bottom up evaluation: better termination, better predictable performance by avoiding recomputation and Well Founded Semantics for negation. Delimited continuations can be used to implement high level new control structures and Engines can be used to control multiple Prolog goals, achieving different control structures such as massive numbers of cooperating agents.

• Nice environment
  SWI-Prolog provides a good command line environment, including‘Do What I Mean', autocompletion, history and a tracer that operates on single key strokes. The system automatically recompiles modified parts of the source code using the make/0 command. The system can be instructed to open an arbitrary editor on the right file and line based on its source database. It ships with various graphical tools and can be combined with the SWI-Prolog editor, PDT (Eclipse plugin for Prolog), VScode or GNU-Emacs.

• Fast compiler
  Even very large applications can be loaded in seconds on most machines. If this is not enough, there is the Quick Load Format. See qcompile/1 and qsave_program/2.

• Transparent compiled code
  SWI-Prolog compiled code can be treated just as interpreted code: you can list it, trace it, etc. This implies you do not have to decide beforehand whether a module should be loaded for debugging or not, and the performance of debugged code is close to that of normal operation.

• Source level debugger
  The source level debugger provides a good overview of your current location in the search tree, variable bindings, your source code and open choice points. Choice point inspection provides meaningful insight to both novices and experienced users. Avoiding unintended choice points often provides a huge increase in performance and a huge saving in memory usage.

• Profiling
  SWI-Prolog offers an execution profiler with either textual output or graphical output. Finding and improving hotspots in a Prolog program may result in huge speedups.

• Flexibility
  SWI-Prolog can easily be integrated with C, supporting non-determinism in Prolog calling C as well as C calling Prolog (see section 12). It can also be embedded in external programs (see section 12.5). System predicates can be redefined locally to provide compatibility with other Prolog systems.

• Threads
  Robust support for multiple threads may improve performance and is a key enabling factor for deploying Prolog in server applications. Threads also facilitates debugging and maintenance of long running processes and embedded Prolog engines. The native IDE tools run in a separate thread The library(prolog_server) library provides telnet access and the pack libssh provides SSH login. With some restrictions regarding the compatibility of old and new code, code can be replaced while it is being executed in another thread. This allows for injecting debug/3 statements as well as fixing bugs without downtime.

• Interfaces
  SWI-Prolog ships with many extension packages that provide robust interfaces to processes, encryption, TCP/IP, TIPC, ODBC, SGML/XML/HTML, RDF, JSON, YAML, HTTP, graphics and much more.