Can you do Logic Programming in Scala?

Question

I read somewhere that Pattern Matching like that supported by the match/case feature in Scala was actually borrowed from Logic languages like Prolog.

Can you use Scala to elegantly solve problems like the Connected Graph problem? e.g. https://web.archive.org/web/20100214144956/https://www.csupomona.edu/~jrfisher/www/prolog_tutorial/2_15.html

Answer 1

No, you can't do it, unless you actually create a logic engine, which kind of defeats the whole purpose.

Furthermore, pattern matching itself is wholly unsuited for this, for many reasons. Consider, for instance, the basic query itself: path(1, 5, P). In Scala's pattern matching, 1, 5 and P are outputs. You can't provide an input that could be used to produce the output.

With Prolog, this is like, assuming that 1 and 5 are fixed, what possible values could P take on? That's just now how pattern matching works.

Edit: With Scala 2.10, pattern matching is now compiled to an intermediate operation like for-comprehensions are, and then the default translation is further optimized. However, it is possible to define your own class to handle pattern matching, and I have seen it used to implement prolog login -- though I can't find the link, sorry.

Answer 2

Haskell and Functional Programming Languages have been the direct source of inspiration for Scala. One of the applications inherited from those languages is the development of Domain Specific Languages (DSLs). There are quite a few DSLs for Logic Programming (Prolog) in Haskell. It should be quite possible to port such a DSL library to Scala, if that has not happened yet.