You can change the clause database in Prolog by asserta/assertz/retract, but that runs headfirst into the typical trade-offs involving mutable state, as does yo

You can change the clause database in Prolog by asserta/assertz/retract, but that runs headfirst into the typical trade-offs involving mutable state, as does your Javascript example. Erlang also lets you change patterns by hot-loading code, but its design goes to great pains to do so with reasonable safety. Also, pattern matching doesn't require all of the variables to be bound. A pattern may specify some values but just partially specify (a list with at least one remaining value) or constrain (any positive integer) others. This makes it vastly more flexible than just dispatching based on one value.