Programming languages without garbage collection send us down a long path of design decisions that lead to slow compile times and fragile runtime performance cliffs.
First is the abandoning of covariance and contravariance, the property which guarantees sensible subtyping: that bytes are also integers, and integers are also objects, extending systematically to container types and functions.
Without garbage collection, offering an array of bytes where an array of integers is required requires stack-allocating an array of integers and converting each byte to an integer every time a subtype is used in place of an actual type. This is so absurd that it’s not done.
So now when we want to recover performance, we need to write all containers and their operations using an increasingly elaborate set of templates or generic functions, which the compiler must specialize for each type at significant cost. This is what C++ and Rust do.
Or we can create a very clunky wrapper like array_of_anything that is used wherever generic types are required, which manually casts and converts values among types dynamically each time it’s accessed. Java generics did this and they were awful.
But if we have garbage collection, we can store our large data structures once with whatever type is required, then dynamically create wrappers that reinterpret it as any subtype that’s required. We pay the cost of GC and indirect control flow for accessors but that’s all.
Unfortunately, most languages missed this opportunity. The C family including C# and Java are overly imperative and lost variance due to wrongly-scoped mutability. And functional languages have generally chosen type systems lacking subtypes, covariance, and contravariance.
