Java-based lightweight Prolog virtual machine
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README.md

README.md

License: Apache 2.0

  • tested with Java 8, developed with IntelliJ and cmd line

  • it assumes SWI-Prolog 7.x for compiling from .pl to .pl.nl files

usage:

  • go.sh add
  • go.sh perms
  • go.sh queens

or

  • go.sh < any pure Prolog program in directory prog >

with Prolog files assumed having suffix .pl

see

  • doc/paper.pdf (A Hitchhiker's Guide to Reinventing a Prolog Machine)

and

for some motivations for this and justification of implementation choices

a swi prolog script first compiles the code to a pl.nl file, than Main calls stuff in Engine which loads it in memory and runs it

Notes

primitive types:

  • int
  • ground
  • var (U+V)
  • ref
  • array

see main code (~1000lines) in Engine.java

  • pl2nl.pl compiles a .pl file to its .nl equivalent, ready to run by the java based runtime system

  • natint.pl emulates its work by compiling back .nl files to Prolog clauses

todo:

  • design a self-contained compiler
  • convert to C for more effective benchmarking against C-based Prolog systems
  • faster runtime: memory efficiency - eg recursive loop, LCO code to be mem_copied with ptrs to var/ref cells to be relocated?

some out of the box thoughts:

  • no symbol tables: a symbol is just a (small) ground array of ints, and instead of a symbol table we would have a "ground cache" - that helps with better memory usege and also speed

  • when a non-ground compound tries to unify with a ground, the ground term is expanded to the heap

  • when a ground unifies with a ground - it's just pointer equality and when a var unifies with a ground, it just points to it - as if it were a constant

Enjoy,

Paul Tarau

August 2017