I want to have a protocol with 3 methods with 2 optional having the following logic:
defprotocol MyRepo.Render do
@fallback_to_any true
def to_string(data)
def to_index_string(data)
def to_show_string(data)
end
# some custom implementations
defimpl MyRepo.Render, for: Any do
def to_index_string(data), do: MyRepo.Render.to_string(data)
def to_show_string(data), do: MyRepo.Render.to_string(data)
def to_string(data), do: MyRepo.Render.to_index_string(data) || "#{inspect data}"
end
So that I can have for some data structures:
defimpl MyApp.Render, for: __MODULE__ do
def to_index_string(data) do
"get something"
end
def to_show_string(booklet_1) do
"get something else"
end
end
And for others:
defimpl MyApp.Render, for: __MODULE__ do
def to_string(data) do
"get something general"
end
end
It seems like for modules with no def impl is present, it works as expected, but when I define either to_index_string and to_show_string or to_string, the compiler complains that the other methods are missing:
** (UndefinedFunctionError) function MyRepo.Render.MyApp.MyDataStruct.to_show_string/1 is undefined or private, but the behaviour MyRepo.Render expects it to be present. Did you mean one of:
* to_string/1
(vae) MyRepo.Render.MyApp.MyDataStruct.to_show_string(%MyApp.MyDataStruct{})
You can do this explicitly with relative ease by just defdelegateing the functions you don’t want to implement to the Any implementation. EG: defdelegate to_index_string, to: MyRepo.Render.Any.
Yep mine can do it. ProtocolEx supports not just implementation on specific types but even specific matchspecs so you could match even on specific values if you want (with a full priority system), along with features of being able to inline the implementations into a single module if you need absolute speed, supports optional callbacks, fallback callbacks, a testing structure, etc… And of course all of the extra stuff is optional.
But yeah, if all you need is ‘just’ an optional kind of thing, defdelegate is easy enough to use at each site, which can be wrapped up in a use with override and so forth perhaps?