I can't find any info on this online... I am also new to Prolog...
It seems to me that Prolog could be highly concurrent, perhaps trying many possibilities at once when trying to match a rule. Are modern Prolog compilers/interpreters inherently* concurrent? Which ones? Is concurrency on by default? Do I need to enable it somehow?
* I am not interested in multi-threading, just inherent concurrency.
In theory that seems attractive, but there are various problems that make such an implementation seem unwise.
for better or worse, people are used to thinking of their programs as executing left-to-right and top-down, even when programming in Prolog. Both the order of clauses for a predicate and of terms within a clause is semantically meaningful in standard Prolog. Parallelizing them would change the behaviour of far too much exising code to become popular.
non-relational language elements such as the cut operator can only be meaningfully be used when you can rely on such execution orders, i.e. they would become unusable in a parallel interpreter unless very complicated dependency tracking were invented.
all existing parallelization solutions incur at least some performance overhead for inter-thread communication.
Prolog is typically used for high-level, deeply recursive problems such as graph traversal, theorem proving etc. Parallelization on a modern machines can (ideally) achieve a speedup of n for some constant n, but it cannot turn an unviable recursive solution method into a viable one, because that would require an exponential speedup. In contrast, the numerical problems that Fortran and C programmers usually solve typically have a high but quite finite cost of computation; it is well worth the effort of parallelization to turn a 10-hour job into a 1-hour job. In contrast, turning a program that can look about 6 moves ahead into one that can (on average) look 6.5 moves ahead just isn't as compelling.