Macros to define and implement interfaces, to ensure they are checked and correct.
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Updated Last
1 Year Ago
Started In
July 2022


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Macros for defining the required behaviours of Julia interfaces, and stating that an object implements them.

The goal is to get as much as possible out of defining an interface, specifically:

  • Traits: All @implements declarations produce compile-time traits that can be checked by other packages - for the whole interface and all of it's optional components.
  • Tests: @implements declarations should be tested in package tests.
  • Docs: interface documentation can be inserted into trait documentation.

Note: the syntax here is likely to change over 2022 as we work out the best ways to define interfaces


See the IterationInterface in BaseInterfaces.jl (a subpackage of this package) for examples of @interface and @implements.

But heres an examples using Animals, and the implementation of a Duck.

First we define the interface methods, and a list of mandatory and optional properties of the interface, with conditions, using the @interface macro.

The @interface macro takes two argumens

  1. The name of the interface, which should usully end with "Interface"
  2. The mandatory and optional components of the interface written as a NamedTuple, with functions or tuple of functions that test them.
module Animals

using Interfaces

# Define the methods the interface uses
function age end
function walk end
function talk end
function dig end

# Define the interface conditions
@interface AnimalInterface (
    mandatory = (;
        age = (
            x -> age(x) isa Real,
            x -> age(x) >= 0,
    optional = (;
        walk = x -> walk(x) isa String,
        talk = x -> talk(x) isa Symbol,
        dig = x -> dig(x) isa String,


Now we can implement the AnimalInterface, for a Duck.

The @implements macro takes three arguments.

  1. The interface type, with a tuple of optional components in its first type parameter.
  2. The the type of the object implementing the interface
  3. Some code that defines an instance of that type that can be used in tests.
using Interfaces

# Define our Duck object
struct Duck

# And extend Animals methods for it
Animals.age(duck::Duck) = duck.age
Animals.walk(::Duck) = "waddle" = :quack

# And define the interface
@implements Animals.AnimalInterface{(:walk, :talk)} Duck [Duck(1), Duck(2)]

Now we have some methods we can use as traits, and test the interface with:

julia> Interfaces.implements(Animals.AnimalInterface{:walk}, Duck)

julia> Interfaces.implements(Animals.AnimalInterface{:dig}, Duck)

# We can test the interface
julia> Interfaces.test(Animals.AnimalInterface, Duck)

# Or components of it:
julia> Interfaces.test(Animals.AnimalInterface{(:walk,:talk)}, Duck)

# Test another object
struct Chicken end

julia> Interfaces.implements(Animals.AnimalInterface, Chicken()) 

If you think it should behave differently or there is better syntax, please make an issue.