Comment by DeathArrow

Others have more complete answers, but the value for me of learning Prolog (in college) was being awakened to a refreshingly different way of expressing a program. Instead of saying "do this and this and this", you say "here's what it would mean for the program to be done".

At work, I bridged the gap between task tracking software and mandatory reports (compliance, etc.). Essentially, it handles distributing the effective working hours of workers across projects, according to a varied and very detailed set of constraints (people take time off, leave the company and come back, sick days, different holidays for different remote workers, folks work on multiple stuff at the same time, have gaps in task tracking, etc.).

In the words of a colleague responsible for said reports it 'eliminated the need for 50+ people to fill timesheets, saves 15 min x 50 people x 52 weeks per year'

It has been (and still is) in use for 10+years already. I'd say 90% of the current team members don't even know the team used to have to "punch a clock" or fill timesheets way back.

Prolog's constraint solving and unification are exactly what is required for solving type-checking constraints in a Hindley-Milner type system.

Any kind of problem involving the construction, search or traversal of graphs of any variety from cyclic semi-directed graphs to trees, linear programming, constraint solving, compilers, databases, formal verification of any kind not just theorem proving, computational theory, data manipulation, and in general anything.

Scheduling, relational modeling, parsing. These things come up all the time. Look at DCG:s if you want to quickly become dangerous.

See my comment here - https://news.ycombinator.com/item?id=45902960