Pattern Matching

match is the preferred branching tool for tagged unions and other value-shape decisions.

Simple example

describe = (value: Number | String): String => match (value) {
  0 => 'zero'
  'ok' => 'status ok'
  _ => 'something else'
}

Tagged union example

type User =
  | { tag: 'guest' }
  | { tag: 'member', name: String }

greet = (user: User): String => match (user) {
  { tag: 'guest' } => 'hello guest'
  { tag: 'member', name } => 'hello ' + name
}

Why match matters

• it makes value shape explicit
• it narrows types inside each arm
• it encourages exhaustive handling of tagged unions

Comparison to TypeScript

TypeScript often uses switch plus ad hoc narrowing checks. FScript gives pattern matching a more central role, which tends to make union-heavy code shorter and clearer.