lazy.hrl

%%% Macro definitions for lazy (or rather delayed) evaluation.

%% Force and delay are used in combination to delay evaluation to only
%% when strictly needed:
%%  Promise = ?delay(expensive_computation(A,B,C)),
%%  Value = ?force(Promise).
%%
%% These macros should be used with extreme precaution when sending a
%% function to another process as it relies on closures and the sharing of
%% information is lost when copying. So is memoization. Therefore, lazy
%% evaluation of that kind should be kept for a single process.
-define(delay(E), fun() -> E end).
-define(force(F), lazy_memoize(F)).

%% memoization uses a process dictionary. While hackish and not portable
%% through processes, a PD of this kind has the advantage of being
%% garbage collected and being returned in a stack trace. There is also no
%% copying (see: comments over the macros) or locking needed.
-define(memoize(E), lazy_memoize(fun()-> E end)).
lazy_memoize(F) when is_function(F) ->
    case erlang:get(F) of
        undefined ->
            erlang:put(F,F()),
            erlang:get(F);
        X -> X
    end.