Hi Josè, thanks for your reply.
Sorry I have done a poor job explaining myself. What I meant I'd love to
have is:
- avoiding "public" write/read direct access to fields in a struct
- forcing users to use an API to "construct" a new struct
I'm pretty sure with these changes achieving the goal "valid struct at
'all' times" would be much easier.
I understand enforcing types for struct fields would be pretty complex.
Besides, this could be worked around wrapping even simple fields (binaries
or integers) into structs (someone said value objects?).
Referring to your example:
def bad(user) do
%{user | name: 5}
end
I don't think this is a problem. That is what in OOP I would call a "dirty
public setter", which can obviously be used, but it should not. Monitoring
against these kind of bad practices is easier anyway.
Also, having "private" structs will give Elixir community a big message
such as: "we care about information hiding".
On Wednesday, 4 October 2017 10:08:17 UTC+1, Maciej Kaszubowski wrote:
>
> Hello,
>
> *Proposed feature*
>
> I'd like to propose another improvement on structs. Inspired by
> @enforce_keys,
> I'd like to propose adding @guards which can help to validate the types of
> the fields in the struct.
>
> Example usage:
>
> defmodule MyStruct do
> @guards [name: :is_binary]
> defstruct [:name]
> end
>
>
> which will fail if the given condition is not satisfied:
>
> iex(2)> %MyStruct{name: 5}
> ** (ArgumentError) The following fields didn't match the guards: struct
> MyStruct: [{:name, :is_binary, 5}]
> expanding struct: MyStruct.__struct__/1
>
>
> *Notes*
>
> - As the example shows, the behaviour will be similar to @enforce_keys
> - it will be checked only when creating the struct, not when updating
> - Using module attribute allows to keep this optional and allows to
> keep backwards compatibility
>
> *Possible implementation*
>
> With https://hexdocs.pm/elixir/master/guards.html and Kernel.apply/3, we
> can modify existing def __struct__(kv) from Kernel:
>
>
> def __struct__(kv) do
> {map, errors} =
> Enum.reduce(kv, {@struct, {[], @enforce_keys}}, fn {key, val}, {map,
> {type_errors, key_errors}} ->
>
> guard = @guards[key]
> if guard && apply(Kernel, guard, [val]) do
> {Map.replace!(map, key, val), {type_errors,
> List.delete(key_errors, key)}}
> else
> {
> Map.replace!(map, key, val),
> {[{key, guard, val} | type_errors], List.delete(key_errors, key)}
> }
> end
>
> end)
> case errors do
> {[], []} -> map
> {types, []} ->
> raise ArgumentError, "The following fields didn't match the guards:
> " <>
> "struct #{inspect __MODULE__}: #{inspect types}"
> end
> end
>
>
>
> This, of course, needs style improvements (and validation of required
> fields which is currently removed for the sake of clarity), but this is
> only a proof of concept to verify that the implementation is possible and
> quite easy.
>
> *Why not use @type?*
>
> While it would be cool to be able to verify the types based on typespecs,
> it would be harder because I think not all types can be easily validated.
> The suggested approach with guards will be feel more familiar because we
> can already do this for functions. Adding guard validation for struct
> fields feels like reasonable step.
>
> *What do you think?*
>
> I'd be happy to start working on this feature, but I wanted to know what
> do you all think about this.
>
>
> Cheers,
> Maciej
>
>
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