You can look at Record, which would remove a bunch of the boilerplate and give you a common API.
LostKobrakai
1
Fl4m3Ph03n1x
I checked out Record and tried to use it, but unfortunately it has one fundamental flaw for this specific use case:
- It requires every field has a default value
So for example:
defmodule Genre do
require Record
Record.defrecord(:genre, :name)
@type t :: record(name: :hard_rock | :heavy_metal | :pop)
end
Won’t compile, because :name has no default.
you could argue “Just use nil as a default”, but I really don’t want that to be possible. In this case, for example, a genre can be 1 of three things, nil is not one of them.
However because something like this is possible:
import Genre
# To create records
record = genre() # this should not be possible
record = genre(name: :hard_rock) #=> {:name, :hard_rock}
The idea falls apart. In contrast, with the previous approaches, dialyzer would pick up such cases and report them as incorrect.
This is unfortunate, as this is the almost perfect solution for the NewType abstraction I am looking for.
0
garrettmichaelgeorge
This is very interesting. Is a zero-cost NewType abstraction similar to a value object in OOP?
- Value Object (Martin Fowler)
- https://en.wikipedia.org/wiki/Value_object
1
aziz
I thought @opaque types were intended for this purpose. So I tried the following example but unfortunately it seems it’s not working and both Dialyzer and Elixir-LS don’t report a warning:
@opaque artist :: binary
@opaque title :: binary
@spec artist!(binary) :: artist
def artist!(a), do: a
@spec title!(binary) :: title
def title!(t), do: t
@spec create(artist, title) :: %{name: artist, title: title}
def create(name, title), do: %{name: name, title: title}
def test() do
# Wrong order of arguments, but no warning/error. :(
create(title!("Title"), artist!("Artist"))
end
1
03juan
There was a discussion recently about changing the way dialyser matches on specs. Perhaps it might help you here?
0
NobbZ
Of course not, everything happens in the same module, and the module that defines an @opaque type has access to it’s internals, and is therefore allowed to use any binary without explicitly “converting” it.
2
Fl4m3Ph03n1x
No, NewType is more like a new primitive type, like Strings or Integers. Value objects are a different concept not related to algebraic data types.
Interesting read though!
2
Fl4m3Ph03n1x
That discussion is not about “changing dialyzer”, but more about finding flags to use in order to detect specific error cases.
This discussion is geared more towards finding a cheap way to define a new type in Elixir. Since it is a new type, it means that dialzyer’s default algorithm would always be able to find incorrect invocations, without the need for underspec or overpsec flags.
1
tomekowal
I’d go with @aziz solution. I usually define the API for other modules to consume so it doesn’t hurt that much that the solution does not work in the same module. It works in other modules:
defmodule Title do
@opaque t :: binary
@spec new(binary) :: t()
def new(t), do: t
end
defmodule Artist do
@opaque t :: binary
@spec new(binary) :: t()
def new(a), do: a
end
defmodule Song do
@opaque t :: %{artist: Artist.t(), title: Title.t()}
@spec create(Artist.t, Title.t) :: t()
def create(artist, title), do: %{artist: artist, title: title}
end
defmodule Test do
alias Title
alias Artist
alias Song
def test() do
title = Title.new("Title")
artist = Artist.new("Artist")
# Wrong order of arguments causes Dialzyer error
Song.create(title, artist)
end
end
mix dialyzer produces
lib/zero_cost.ex:24:no_return
Function test/0 has no local return.
________________________________________________________________________________
lib/zero_cost.ex:29:call_without_opaque
Function call without opaqueness type mismatch.
Call does not have expected opaque terms in the 1st and 2nd position.
Song.create(_title :: Title.t(), _artist :: Artist.t())
________________________________________________________________________________
done (warnings were emitted)
Halting VM with exit status 2
Rafał Studnicki used this idea in one of his projects: https://www.youtube.com/watch?v=XGeK9q6yjsg He goes even further and with those types 
However, there is some boilerplate involved and with dialyzer cryptic errors, I don’t see this solution getting too much traction
8
aziz
That’s cool, @tomekowal! 
I wonder if nested modules would work? I’m sure the boilerplate code can be reduced significantly with macros. And the dialyzer error messages being cryptic… well, our brains get used to it after some time and we don’t try to understand the words anymore and just take the line number and try to fix the issue there 
. (Of course, if you never started out with dialyzer it’s gonna be tough. 
)
Thanks for the pointer to video!
Edit: Yep, it works also when you move the Title & Artist modules into the Song module. 
0