Traits in Wipple are implemented!

After months of work, I’ve finally implemented the Wipple trait system! In this post, I want to explain how to use traits in Wipple programs.

In the same way that a type represents a relationship between unique values (1 :: Number, 2 :: Number, 3 :: Number, etc.), a trait represents a relationship between unique types. Let’s say we have a function called show that displays its input on the screen. Ideally, we would be able to provide a value of any type to show, as long as the value is “showable”. We can express this idea using where:

show :: A where (Show A) => A -> ()

What is Show? That’s our trait! Let’s define it:

Show : A => trait (A -> Text)

This piece of code means “for any type A, to Show an A means to implement a function that, given a value of type A, produces a Text”.

Now let’s do just that — implement the function for our various types!

instance Show Text : it -- text is already text


Foo : type

instance Show Foo : just "foo"


Person : type {
    name :: Text
}

instance Show Person : { name } -> format "A person named _" name

And now we can call show with values of these types:

show "Hello, world!" -- Hello, world!
show Foo -- foo
show (Person { name : "Bob" }) -- A person named Bob

All in all, I’m very proud of how this turned out! The Wipple typechecker has been one of the most complex pieces of software I’ve ever written, and I’m glad to be able to finally turn my attention to other parts of the language. Next up is pattern matching and templates/operators — I’ll come back to higher-kinded types someday if they prove necessary. Until next time!