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Literal arrays edge ​

TIP

This features is not yet released. You can see the current in-progress implementation here and follow the issue here. Contributions are very welcome.

The _Array(T) type can be used to check that an object is an Array and all of its elements match the type T, but after checking that, there’s nothing to prevent you from unintentionally inserting an invalid element.

Using _Array(T) is a good start, but you can take take things further. Literal::Array(T) is an object that behaves like an Array but continues to maintain the type T throughout its lifetime.

Creating a Literal array ​

You can create a Literal::Array(T) like this:

ruby
array = Literal::Array(String).new

You can also pass in an initial set of elements:

ruby
array = Literal::Array(String).new("Hello", "World")

Mapping ​

When mapping a Literal::Array(T), you need to pass in the type for the new Literal::Array.

ruby
array = Literal::Array(String).new("Hello", "World")

mapped = array.map(Integer) do |it|
  it.length * 10
end

mapped here will be a Literal::Array(Integer) and will have checked the types of each element in the mapping. An invalid mapping will raise a Literal::TypeError.

If you map from one basic type to another basic type using a Symbol proc, Literal may be able to skip the type check.

ruby
array = Literal::Array(String).new("Hello", "World")
mapped = array.map(Integer, &:length)

In this example, Literal can see that &:length will call #length on a String and it knows that method always returns an Integer.

Covariance ​

If both types are modules (or classes), Literal::Array generics support covariance. This means a Literal::Array(Integer) is a subtype of Literal::Array(Numeric).

Beyond basic module and classes, some of Literal’s built in types support covariance and we’re working on adding more. For example, Literal::Array(true) is a subtype of Literal::Array(_Boolean).