domain-modeling-made-functional

Model each state as its own sealed record in a discriminated union hierarchy, carrying only fields valid in that state. Wrap primitives like Guid and string in single-case record types with validated factory methods so invalid values cannot be constructed.

Return domain errors as values in an Either or Result type with a sum type of named failure cases, since expected rejections are part of the contract. Reserve exceptions for infrastructure failures propagated as effects and true panics from programmer bugs.

The impureim sandwich structures an application service as impure read, pure decision, impure write. The shell gathers all inputs, calls one pure domain function that returns decisions or events, then persists and publishes the results.

These techniques rely on a compiler to enforce type-encoded invariants, so they do not apply to Python, Ruby, or vanilla JavaScript. Use this approach in F#, Scala, Kotlin, TypeScript, or C# with records and LanguageExt.

First try hoisting the read to the front of the workflow. If that is impossible, split into two shell-orchestrated sandwiches where the pure core returns a value like NeedsReprice and the shell performs the fetch, reserving free monads for genuine multi-round interleaving.

Use a sum type when a record has multiple Option fields or sibling-exclusive booleans where only some combinations are legal. Each legal combination becomes its own case, so the compiler forces callers to handle every state and forbids invalid ones.