How does one use cached data in a functional language such as Erlang?

Question

I\'ve been reading a bit lately about functional languages. Coming from 10+ years of OO development, I\'m finding it difficult to get my head around how on earth one can poi

Answer 1

It is data which must be immutable in Erlang, not actors.

Long-lived actors normally live in a tail-recursive function, the arguments of which serve as their state and certainly can change between calls.

-module(cache).
-export([start/0, get_c/1, put_c/2, clear/1]).

start() -> register(spawn(fun () -> loop(dict:new()) end), cache).

loop(Dict) -> receive
                {get, From, Key} -> From ! {cache_result, Key, dict:fetch(Key, Dict)};
                {set, Key, Value} -> NewDict = dict:store(Key, Value, Dict),
                                     loop(NewDict);
                %% etc.
              end

put_c(Key, Value) -> cache ! {set, Key, Value}
%% etc.

When you call put_c, the actor's "state" changes even though all data involved is immutable.

Answer 2

Memoize the function. A cache is just a list/dictionary, and hence can be implemented in a purely functional way.

Answer 3

There is no reason a Cache and a Functional language can't live together. To be functional you just have to obey the constraint that calling the same function with the same arguments you get the same answer.

For instance: get_data(Query, CacheCriteria)

Just because the get_data uses a cache doesn't mean it's not functional. As long as calling get_data with the same Query, and CacheCriteria arguments always returns the same value then the language can be considered functional.