Learn

match is OCaml's switch — but it matches by structure, binds variables, and is checked for exhaustiveness:

let describe n =
  match n with
  | 0 -> "zero"
  | n when n > 0 -> "positive"
  | _ -> "negative"

Patterns:

• 0 — literal
• n — bind to name (catches everything from there on)
• n when ... — guard
• _ — wildcard
• (a, b) — tuple
• [], [x], [x; y], x :: xs — list patterns
• Some x, None — option

Lists are central to OCaml. The :: (cons) pattern destructures into head and tail:

let rec sum lst =
  match lst with
  | [] -> 0
  | x :: xs -> x + sum xs

The compiler warns when you miss a case. Pattern matches that don't cover all possibilities are flagged at compile time — a major safety feature.