I was expecting that to be a compile time transformation that would
generate the exact same compiled erlang as
```
@type my_struct_with_a() :: %MyStruct{
a: integer(),
b: integer() | nil,
c: integer() | nil
}
```
But that would require `@type` to be able to get the AST for `MyStuct.t()`
during compile time.
On Fri, Feb 5, 2021 at 8:21 PM Louis Pilfold <[email protected]> wrote:
> Hello!
>
> Correct me if I'm wrong but I believe this isn't something that Erlang
> typespecs support, or at least there is no syntax for it.
>
> What do you see this compiling to? Elixir has to work with the capability
> of Erlang here.
>
> Cheers,
> Louis
>
> On Fri, 5 Feb 2021, 22:39 Bernardo Amorim, <[email protected]> wrote:
>
>> Hi folks, I'm not even sure if this is possible nor if it was raised
>> before nor if there is already another way of doing something similar. But
>> this is something that I've been thinking a lot recently and I'd like to
>> know if it is possible and desirable. If it is, I can help with a PR later
>> on.
>>
>> The "problem":
>>
>> It is usual to when we have a Struct (specially Ecto.Schemas) to define a
>> `@type t()` with the fields. Let's say we have a Struct with a few fields
>> like this:
>>
>> ```
>> defmodule MyStruct do
>> defstruct [:a, :b, :c]
>> @type t() :: %__MODULE__{
>> a: integer() | nil,
>> b: integer() | nil,
>> c: integer() | nil
>> }
>> ```
>>
>> Now imagine I want to define a function that receives a struct but
>> requires one of these fields to be non null. What we have to do right now
>> is to:
>>
>> ```
>> @type my_struct_with_a() :: %MyStruct{
>> a: integer(),
>> b: integer() | nil,
>> c: integer() | nil
>> }
>> ```
>>
>> (Ok, maybe in some cases we do not need to redefine all the other fields
>> since they might be irrelevant, but let's assume we actually want to type
>> everything)
>>
>> The proposal:
>>
>> For map values when we just want to change one field we can do something
>> like: ```%MyStruct{my_struct | a: 1}```, but for types this is not possible.
>>
>> My proposal would be to have something like this:
>>
>> ```
>> @type my_struct_with_a() :: %MyStruct{MyStruct.t() | a: integer()}
>> ```
>>
>> That would generate a type with all fields copied from `MyStruct.t()` but
>> with `:a` changed to `integer()` instead of `integer() | nil`.
>>
>> The caveats I see is what to do when the type is not just a map type
>> (maybe it is a sum type, maybe it is not even a map, etc).
>>
>> What do you folks think about this?
>>
>> Thanks,
>> Bernardo Amorim
>>
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