A similar question was asked Parameterised Modules in Erlang, it is about "what". My question is about "why"?
OTP Technical Board - Decisions affecting R16 contains the board decision about this issue, but I don't know the reason behind the decision.
Stateful Module in Programming Erlang 2ndEdition by Joe Armstrong introduces this feature in detail, but I don't see the author's attitude.
If we read the official document Function Calls, we see this feature is deliberately skimmed. In fact, the official document strongly discourages using this feature, refer to efficiency function calls. If so, why Joe Armstrong mentions such feature in his book?
I think this feature is awesome. As the above book mentioned, my client code could be like below
Obj:find(Key),
Obj:is_key(Key),
Then, we don't care whether Obj
is created by dict:new()
, or gb_tree:new()
, unfortunately, dict
and gb_tree
do not share a consistent interface, e.g. we have gb_tree:lookup
instead of gb_tree:find
.
I can't tell you what the discussion was within the Great Cabal That Controls Everything, but I can tell you a few reasons why I never have considered using this feature:
It introduces syntactic complexity and semantic abiguity but grants me no new superpowers.
Complexity:
X
of Foo
equal to 10 or 20 right now?"dict:is_key(Value, Thingy)
here and then Thingy:is_key(Value)
over there?dict:is_key(Key, Foo:get_value(Key2))
all the time?"Ambiguity:
This introduces opaque state (bad) instead of an ADT (good, and something we already have).
Foo:is_key(Key)
than dict:is_key(Key, Foo)
. Other than the fact that I am certain on first reading, even in a complete absence of context, that the data object being operated upon in the second version is definitely a dict.Erlang's symbol assignment (aka "single assignment") is great, why destroy that?