A side-effect-free programming language?

I am building a system that stores an event log of things that happened, and I would like the users to be able to set some custom functions that would reduce the event log into some sensible "views" of the state.

For that I would like to use an embedded programming language, could be anything but ideally doesn't have any side-effect and simply works with its input data to produce an output.

Such language would be pointless on its own of course, but does someone know if an interpreted script specifically made for embedded programming exists, or if a more "mainstream" interpreter could easily be compiled without the I/O machinery