Writing an emulator for a slightly unusual abstract FSM

what would be a relatively hassle-free way of writing an elegant, intuitive code that emulates a finite state machine (no jumps or conditionals) with a self-modifying instruction set (or maybe just two instruction sets: one for modifying data, other for modifying the first instruction set)? I don't care about performance right now (I doubt it will get to the stage when I need to care about that but if it does, some fuckery with C function pointers will be inevitable; right now I care about readability and being able to prototype quickly) so functional is OK but I'm thinking of instructions having four-bit length and I'm not sure how good purefuncs are at handling that

verilog or vhdl?