Pattern Matching

OCaml

let rec sum = function
  | [] -> 0
  | h :: t -> h + sum t
 
match List.hd [] with
  | [] -> "empty"
  | _ :: _ -> "nonempty"
  | exception (Failure s) -> s

Rust

TypeScript

Use never

type Shape = Circle | Square;
 
function getArea(shape: Shape) {
  switch (shape.kind) {
    case "circle":
      return Math.PI * shape.radius ** 2;
    case "square":
      return shape.sideLength ** 2;
    default:
      const _exhaustiveCheck: never = shape;
      return _exhaustiveCheck;
  }
}

Haskell

-- case of
-- parse country code into country name, returns Nothing if code not recognized
parseCountry :: String -> Maybe String
parseCountry "FI" = Just "Finland"
parseCountry "SE" = Just "Sweden"
parseCountry _ = Nothing
 
flyTo :: String -> String
flyTo countryCode = case parseCountry countryCode of Just country -> "You're flying to " ++ country
                                                     Nothing -> "You're not flying anywhere"