jakub-zawislak
Protocols and pattern matching?
I’m trying to write two independent libraries with the same API. I thought I would achieve it with Protocols. I have already reviewed implementations of Phoenix.HTML.FormData for Plug.Conn and for Ecto.Changeset.
One of my library epxects a simple struct as argument, and the other one an Ecto’s schema. I know how to distinguish those in a pattern matching. The problem is, the defimpl ..., for: ... expects a module as :for parameter, not the pattern.
Is there any way to solve this problem using Protocols?
The only idea I have is to wrap those arguments with another modules like this: %MyLib.Struct{data: struct}, %MyLib.Schema{data: schema}. Then to write implementation of my protocol for MyLib.Struct and MyLib.Schema. But it wouldn’t be programmer-friendly 
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marcelotto
Protocols can’t be dispatched by pattern matching in a native way like Qqwy already mentioned, but ProtocolEx seems to get this to work.
OvermindDL1
Perfectly suited for that.
Fairly stable now, and if you use the suggested names (I have some names that only exist for backwards compatibility that ‘could’ potentially be removed in the future, but probably will not, and it would be a major version bump anyway).
It goes for more power than Elixir traditionally prefers because it is easier to break things than the stock protocol if you do things wrong (though I have a lot of checks and newer/documented styles of doing things that handle much of those issues now).
Hah, really? This is NOT an example of good code internally. It started purely as an experiment of How easy would it be to do something like this... and it indeed up being something I actually ‘use’ and I just never refactored it (refactoring would be done in a backwards compatible way though). ^.^;
OvermindDL1
Ah this is an old thread. ^.^
But yep, that was the purpose of why I made ProtocolEx, and I had a few ideas to get some extra speed out of it like being able to @inline definitions or defmacro implementations into the protocol and such, as well as inline case-by-case default implementations, all of which is implemented, and it can run circles around the stock Protocol in speed and capabilities at this point while entirely replacing its feature set with just a couple extra tokens overall. 
Qqwy
Protocols are in fact a wrapper around Behaviours, that reduce a little boilerplate, but will only work if your input is a struct (or one of Elixir’s built-in datatypes). Dispatching a protocol based on a pattern-match is not possible.
Instead, I’d advise you to just write a behaviour for this.
samphilipd
If one of your implementations has to deal with arbitrary structs then defimpl can’t help you I think. If it’s one particular type of struct then it should be fine.
I tried to demystify protocols a bit and explain the difference between those and behaviours in my post here.
Could you post here the details of the other struct you’re trying to match on? I think you can use protocols for this.
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