Module lambek::type_app [−][src]
Expand description
Traits for the kind of unary type application, Type -> Type.
Higher kinded types (HKT) such as Type -> Type are not natively
supported in Rust. As such, we cannot use type constructors
such as Vec without applying a specific type as an argument,
e.g. Vec<u8>. Although the upcoming generic associated types (GAT)
feature will partially solve this issue, the feature is not yet
stable and may subject to changes.
An alternative approach is to use defunctionalization to encode
regular Rust types to have kinds other than Type. TypeApp
is one such trait for encoding types of kind Type -> Type.
To promote a type constructor such as Vec to HKT, we define a
proxy type VecF and implement TypeCon and TypeApp for them.
We then use VecF as the unapplied version of Vec in our code.
Inside type signatures, we use App<VecF, X> to apply VecF
to a type X. App<VecF, X> is isomorphic to Vec<X>, and
we can convert a value xs: App<VecF, X> back into Vec<X>
by calling xs.get_applied().
The type App encapsulates the TypeApp constraint using
HasTypeApp. With that, type signatures that use App<F, X>
do not need to have TypeApp<F, X> in their trait bounds.
This makes it significantly easier to perform arbitrary type
applications without having to worry about polluting the
trait bounds with TypeApp constraints. See
Functor for a practical use of App.
Structs
Enums
Traits
Encapsulates an applied type into a trait object to prevent
TypeApp constraints from propagating to type signatures.
A type F: TypeApp<X> have the associated type Applied as the
result of applying a type F of kind Type -> Type to X.
A proxy type F: TypeCon to mark itself as having the kind
Type -> Type.