either_field

either_field is a Crate which allows you to create variants of structs where fields have different types

Why would you use this

While normally you can do a similar thing with enums and their variants, nothing stops you from changing a field's enum variant.

Otherwise, using traits, there is a lot of verbosity, which leads to several dozens lines of codes for just a few generic types in a struct.

This crate instead allows you to generate separate types from a (under the hood) generic struct.

E.G:

Picture you're working with SQL, and you have two tables:

• Scores

  	player_id	value
  Type	INTEGER	INTEGER

and

• Players

  	player_id	username
  Type	INTEGER	VARCHAR(255)

Say that in a function, you're returning either the pure Scores table, or a modified version with the player

// Example 1
struct Scores {
    player_id: i32,
    value: i32
}

// Example 2
struct PlayerBasic {
    player_id: i32,
    username: String
}

struct ScoresWithPlayer {
    player: PlayerBasic,
    value: i32
}

Wouldn't it come in handy if you could define both of these with one struct?

#[template_table]
struct GenericPlayer {
    player: either!(i32 | PlayerBasic),
    value: i32
}

The Syntax

Structs

within the #[make_template()] attribute macro inputs, we use a JSON-like syntax to indicate what's going on. Here's a description of it:

• VIS is the visibility of the struct

VIS name_of_the_derived_struct: [
    field_name_1: type,
    field_name_2: type
]

within the struct instead, we use a macro to tell which fields should be considered and what their valid types are:

struct StructName {
    field_name: either_field::either!(type | type | type)
}

The settings

Before the list of generated structs, there can be settings:

SettingName: value,

These are separated by commas, the last one by a semicolon, and the possible values are the following:

Tuple Structs

Tuple structs require for GenStructs to be set to true.

it is allowed to either indicate the fields with a number, or do so sequentially

VIS name_of_the_derived_struct: [
    field_position: type, // i.e. 0
    ...
]

VIS name_of_the_derived_struct: [ type, ... ]

The actual examples are in the example/ folder.