Narrowing and Exhaustiveness

Pattern matching narrows types within each branch.

Example

type LoadResult =
  | { tag: 'loading' }
  | { tag: 'loaded', value: String }
  | { tag: 'failed', message: String }

message = (result: LoadResult): String => match (result) {
  { tag: 'loading' } => 'loading'
  { tag: 'loaded', value } => value
  { tag: 'failed', message } => message
}

Inside each branch, the active variant is known more precisely.

Why exhaustiveness matters

Exhaustive handling helps:

• avoid forgotten cases
• keep union-heavy code maintainable
• support stronger compiler guarantees

Comparison to TypeScript

TypeScript can narrow through control flow too, but FScript leans harder on match and tagged unions as the standard way to express those checks.