proposal: Shorthand syntax to access structs keys values with compile-time checks

[Ulisses De Almeida]

Good evening, morning, afternoon!

I was wondering with my team what is the best way of acessing keys from structs, and we have the dynamic way. That's is something like:

def create_order!(item, user) do
  Repo.insert!(%Order{
    user_id: user.id,
    user_email: user.email,
    user_shipping_address_id: user.user_shipping_address_id,
    total_price: item.price,
    total_weight: item.weight,
    quantity: 1
  })
end

This dynamic way is very short and direct to the point. However, if there's any typo on the key, if we don't have any unit test, it will show up during the runtime.

Elixir then, provide us the pattern matching way:

def create_order!(
      %Item{
        price: item_price,
        weight: item_weight
      },
      %User{
        id: user_id,
        email: user_email,
        shipping_address_id: user_shipping_address_id
      }
    ) do
  Repo.insert!(%Order{
    user_id: user_id,
    user_email: user_email,
    user_shipping_address_id: user_shipping_address_id,
    total_price: item_price,
    total_weight: item_weight,
    quantity: 1
  })
end

We split up in two steps, one to bind the key value to a variable, then another step to build the new struct. This extra step make the code more verbose generating 2*n lines where `n` is the number of attributes you want to assign.

We're discussing how we could make the code shorter and straight to the point like the dynamic version, we thought in something like creating functions to get values from the keys. Something like:

def create_order!(item, user) do
  Repo.insert!(%Order{
    user_id: User.id(user),
    user_email: User.email(user),
    user_shipping_address_id: User.shipping_address_id(user),
    total_price: Item.price(item),
    total_weight: Item.weight(item),
    quantity: 1
  })
end

Using functions like this, we have compile time checks that we didn't make any typo and also is shorter and straightforward like the dynamic version.

The downsinde is creating functions like this, it's a little bit boring and repetitive. I was thinking if there's any space for a shorthad syntax.

Today we have a shorthand syntax to update key values like this:

%User{user | email: "aaaa"}

I wonder if we can have similar syntax to get a value from a key. Suggestions:

%User{user | :email}
%User{user |> :email}
%User{user -> :email}

Then we could write:

def create_order!(item, user) do
  Repo.insert!(%Order{
    user_id: %User{user -> :id},
    user_email: %User{user -> :email},
    user_shipping_address_id: %User{user -> :user_shipping_address_id},
    total_price: %Item{item -> :price},
    total_weight: %Item{item -> :weight},
    quantity: 1
  })
end

What do you folks think? Does it make sense?

--

Ulisses H. F. de Almeida

ulissesa...@gmail.com
ani...@gmail.com

[Wojtek Mach]

When we do:

    def foo(%User{} = user) do

      user.bad_field

    end

The compiler should have all the information it needs to catch invalid fields. (At least until the variable is re-bound but ideally even then.) So I believe there is no need to introduce new syntax!

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[Ulisses De Almeida]

It's true. I assumed this check wasn't happening because of some limitation of the syntax. But, like you said, it does have all the information.

I wonder why it's not happening now. Was it intentional? 

I would like to introduce this check if there's no drawback . However, I don't know where to start. If you folks could give me some orientation where I should start to look at it.

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[José Valim]

We are not checking right now the plan is to check in the future. :) Possibly by 1.12 or 1.13.

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[Ulisses De Almeida]

Amazing ♥️♥️♥️💜💜💚💙🤎💛

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