I'm assuming you're asking what's important in writing "production" Haskell rather than "toy example" Haskell.

Ixiaus's point about mechanics more than theory certainly rings true, though we did think a lot about whether to use GADTs for the Dimension type. Overall I see this as similar to writing "production" code in other languages, going through a couple feedback loops using real use-cases. Profiling to find the bottlenecks, observing how APIs are used in practice compared to intent, and reaching the service to a stable equilibrium.