Unions & case

Records say "this and that". Tagged unions say "this or that" — and they're where Sky's type system really earns its keep.

Defining a union

type Shape
    = Circle Float
    | Rect Float Float
    | Dot

Shape is a type with three variants. Circle carries one Float (a radius), Rect carries two (width and height), and Dot carries nothing. The variant names are also the constructors:

a = Circle 5.0
b = Rect 3.0 4.0
c = Dot

Taking them apart with case

To use a union value you pattern-match on it with case … of, binding the payload to names:

area : Shape -> Float
area shape =
    case shape of
        Circle r ->
            3.14159 * r * r

        Rect w h ->
            w * h

        Dot ->
            0.0

Exhaustiveness is checked

Here's the payoff. If you forget a variant, the program does not compile:

-- ✗ compile error: missing the Dot case
-- area shape =
--     case shape of
--         Circle r -> 3.14159 * r * r
--         Rect w h -> w * h

Add a variant to Shape a year from now, and the compiler walks you to every case that needs updating. No silent fall-through, no forgotten branch.

Patterns nest

Patterns can go deeper than one level and use _ to ignore what you don't need:

describe : Maybe Shape -> String
describe maybeShape =
    case maybeShape of
        Just (Circle _) -> "a circle"
        Just _          -> "some shape"
        Nothing         -> "nothing"

(Maybe is the next lesson — it's just a union that means "a value, or none".)

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