3. Shapes in tiny labelled boxes

type Shape is Data:
  Circle{r: U32}
  Square{s: U32}

This is an algebraic datatype: a Shape is either a circle carrying a radius or a square carrying a side length. Circle and Square are constructors, not subclasses. Square{5} constructs a value.

is Data says values of this type may be copied. It does not make every binding automatically reusable.

To inspect a shape, use pattern matching:

def area(x: Shape) -> U32:
  match x:
    case Circle{+r}:
      (3 * r * r : U32)
    case Square{+s}:
      (s * s : U32)

Each pattern opens one kind of box and names its contents. +r and +s let the selected numeric field be reused. The circle formula deliberately uses a crude approximation of pi: this is a datatype lesson, not a geometry certification.

Haskell companion:
data Shape = Circle Word32 | Square Word32

area :: Shape -> Word32
area (Circle r) = 3 * r * r
area (Square s) = s * s
Both languages describe alternatives with constructors and handle them with patterns. Bend adds a kind (Data here) and explicit reuse annotations. Constructor braces in Bend are construction/pattern syntax, not Haskell record-update syntax.

A small but important matching fence

Current Bend has no if syntax: branch by matching True{} and False{}. It also restricts the thing you match on: use a parameter or a pattern-bound variable, in binder order. You cannot directly write match area(shape):, nor introduce a local computation before matching a parameter.

For a computed decision, pass the computed value to a helper that matches its parameter. Haskell's unrestricted case expression of is not a safe syntax guide here.

Try it: change main to area(Circle{4}). Expect 48. Then add a Rectangle{w: U32, h: U32} constructor and its matching branch.