What does %Mod{} really do?

Questions & Help Questions
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1

Hello,

given the files xx.ex and yy.ex defined as

defmodule XX do
   defstruct [:field]
end

defmodule YY do
  defdelegate __struct__, to: XX
  defdelegate __struct__(x), to: XX
end

why does

iex(1)> %XX{} == XX.__struct__()
true

and

iex(2)> %YY{} == YY.__struct__()
false

give different results?

It seems that %Mod{} is not equivalent to Mod.__struct__() …

What does %Mod{} really do?

Thanks
Mário

2 
iex(14)> Map.to_list(%YY{})                
[__struct__: YY, field: nil]
iex(15)> Map.to_list(YY.__struct__())
[__struct__: XX, field: nil]

Kernel.SpecialForms.%/2

Based on the evidence I suspect that the compiled code (it’s a special form after all) sanitizes the value returned from the function by forcing the __struct__ value to reflect the primary module - but that is just a guess.

3

My question is this behavior some kind of internal “black magic”, as you suggest, or a bug …

Isn’t %Mod{} supposed to be the same as calling Mod.__struct__()?

4

I kind of suspect this might be a bug, as per this evidence:

this seems ok

iex(2)> struct!(XX, field: 1)
%XX{field: 1}
iex(3)> %XX{field: 1}
%XX{field: 1}

this seems wrong

iex(4)> struct!(YY, field: 1)
%XX{field: 1}
iex(5)> %YY{field: 1}
%YY{field: 1}

Does anyone have an explanation for this, or is this a bug?

If %Mod{} is supposed to be the same as calling Mod.__struct__() or struct!(Mod), then it is a bug.

Thanks

5

I doubt it’s a bug. For example the Access Behavior makes use of that __struct__ value to access the structure specific implementations for navigation and access. If __struct__ was left at XX you would automatically “inherit” all the XX functionality but have no way of overriding it for YY.

6

But then the behavior of struct!(YY, ...) should also be to return an %YY{...}, but it is returning an %XX{...} …

There is some inconsistency somewhere, hence a bug.

7 
iex(16)> struct!(YY,field: 1)
%XX{field: 1}
iex(17)> struct!(%YY{},field: 1)
%YY{field: 1}

I suspect the bug is with Kernel.struct!/2, not Kernel.SpecialForms.%/2.

https://github.com/elixir-lang/elixir/blob/v1.8.1/lib/elixir/lib/kernel.ex#L2116-L2130

As you discovered, it only presents when you start defdelegating the constructor and I don’t think this code anticipated that.

So the question is - what are you trying to accomplish by defdelegating the struct constructor to another struct? Sharing code between XX and YY should probably be accomplished via a shared third module.

8

This issue has been solved here.