[erlang-questions] Parameterized module idioms

Garrett Smith <>
Sat Apr 17 03:34:58 CEST 2010


On Fri, Apr 16, 2010 at 8:04 PM, Bob Ippolito <> wrote:
> On Fri, Apr 16, 2010 at 5:13 PM, Fred Hebert <> wrote:
>> On Fri, Apr 16, 2010 at 8:06 PM, Bob Ippolito <> wrote:
>>>
>>> Technically speaking the difference is:
>>>
>>> E:echo("hello") vs. some_module:echo(E, "hello")
>>>
>>> The biggest reason I've seen to use parameterized modules is that you
>>> could have a different module that implements the same interface, e.g.
>>> sets and gb_sets and you could change which one you're using without
>>> changing any of the code that consumes it.
>>>
>>> I would definitely change the original code to use two modules, one
>>> module to do the start_link stuff, and another module to actually
>>> implement the interface.
>>>
>> M = sets,
>> D = M:new().
>> M:add_element(10, D).
>>
>> This works as well without a problem. Switch the value of M and if the
>> interface is the same, nothing changes.
>
> Yes, but now you're passing around two variables instead of one.

I'd be tempted to create a wrapper that took the impl module in its
new function and maintained it as internal state.

-module(gen_dict).

-define(state, {impl, impl_state}).

new(Impl) ->
    #state{impl=Impl, impl_state=Impl:new()}.

store(Key, Val, #state{impl=Impl, impl_state=ImplState}=S) ->
    S#state{impl_state=Impl:store(Key, Val, ImplState)}.

... and so on...

And to use (obviously):

D = gen_dict:new(dict),
D2 = gen_dict:store(foo, bar, D)

Granted, there's a fair amount of horsing around, but not terrible.
And your generic API is explicit.

Behaviors should work for this as well as they just enforce an exported API.

Garrett


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