Sebb
Struct without fields
I just tried to create a struct without fields (because: why not?).
It’s not possible with defstruct, which insists on an argument.
Best it can do is
defstruct _: nil # nice
btw, this is not OK:
defstruct [_]
isn’t that inconsistent?
So I tried to figure out what defstruct does. From the docs:
To define a struct, a developer must define both
__struct__/0and__struct__/1functions.defstruct/1is a convenience macro which defines such functions with some conveniences.
Where are __struct__/x documented?
Structs are usually defined with the
Kernel.defstruct/1macro
What if I want to be unusual? How can I define a struct by hand, maybe even without fields?
Marked As Solved
NobbZ
Using an empty list as argument to defstruct/1works for me:
iex(1)> defmodule S do
...(1)> defstruct []
...(1)> end
iex(2)> %S{}
%S{}
Also Liked
aiwaiwa
One legitimate reason would be to implement a protocol without data.
Use case: postgrex/lib/postgrex/parameters.ex at v0.16.5 · elixir-ecto/postgrex · GitHub
dimitarvp
sheharyarn
Like @Nobbz said, you can just do this:
defmodule X, do: defstruct []
The reason this doesn’t work is that you cannot assign _. The first example works because :_ is not _, so you can do defstruct [:_] which is equivalent to your first example.
This is the macro that’s called underneath when defining a new struct. Going over the definition should help you better understand manually defining a struct in a module.
Last Post!
aiwaiwa
One legitimate reason would be to implement a protocol without data.
Use case: postgrex/lib/postgrex/parameters.ex at v0.16.5 · elixir-ecto/postgrex · GitHub
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