Subtyping Without Reduction

Subtypes are useful and ubiquitous, allowing important properties of data to be captured directly in types. However, the standard encoding of subtypes gives no control over when the reduction of subtyping proofs takes place, which can significantly impact the performance of type-checking. In this article, we show how operations on a subtype can be represented in a more efficient manner that exhibits no reduction behaviour. We present the general form of the technique in Cubical Agda by exploiting its support by higher-inductive types, and demonstrate the practical use of the technique with a number of examples.