Union Types and Option Types

A closer look at Union Types and Option Types

What this post is about

This is a blog post describing experiments with union types and, more specifically, option types in the author’s low-level language called Copper. It explains what union types are, why the author found the general union feature too complex for Copper, and motivates focusing on the option type (T|Nothing or T?).

What you can learn or do from the post

Read how the author implemented option types in Copper by reserving a special nil value inside the original type, with examples for pointers and other concrete representations (pointer 0, a reserved integer like -1, or a reserved 32-bit pattern for colors). The post details trade-offs: good interoperability with C, efficiency (no tag), flexibility, but issues when a type has no reservable nil value and the impossibility of nested option types (T??). It also covers refinement patterns, checks, a runtime notNil helper for debug, method definitions on optional types, macros like ifNil, and compiler-based inference of non-nil locals (similar to TypeScript/Kotlin) with some analysis limitations on loops.

Who will find it useful

Language implementers and systems-language programmers interested in low-level, tagless option/nullable representations will get concrete ideas and trade-offs. People working on interop with C or designing compact option semantics for primitive and pointer types will find the implementation notes practical. It is less relevant if you need a fully specified language feature or formal semantics beyond the author’s implementation and experiences.

Questions people ask about Union Types and Option Types

Does the post explain how nil is represented for different types?

Yes: it describes reserving a nil value per type, with examples like address 0 for pointers, -1 for signed indices, or a reserved bit pattern in an integer holding RGB color data.

Will Copper allow nested option types like T???

The post explicitly says T?? is not possible with this implementation approach.

Does the author add runtime checks for unsafe casts from T? to T?

The author did not change the language cast operator but added a standard-library notNil method that performs a runtime check in debug mode; other approaches include compiler inference and explicit check constructs discussed in the post.

Union Types and Option Types was shared on NexURLs by Nexis on November 3, 2026, in Learning & Books.