match, exhaustively

You’ve met match on scalars and on optionals. Here it does its real job: opening a union, pulling the fields out of each variant, and forcing you to handle every case.

type Shape =
  | Circle{ radius: Float }
  | Rect{ width: Float, height: Float };
fix first = Rect{ width: 3.0, height: 4.0 };
match first {
  Circle{ radius } -> radius * radius * 3,
  Rect{ width, height } -> width * height,
}

Each arm names a variant and binds its fields — radius in one, width/height in the other — so you’re always working with exactly the data that case carries.

Miss a variant and Ascent won’t let it slide:

type Shape =
  | Circle{ radius: Float }
  | Rect{ width: Float, height: Float };
fix c = Circle{ radius: 2.0 };
match c {
  Circle{ radius } -> radius,
}

“This match doesn’t handle Rect” — flagged on the spot, Run greyed out until you add the arm. For a union you list every variant and skip the catch-all; that way, adding a new variant later makes the compiler come find you.