Type Aliases

type Meters = float and friends — transparent aliases for clearer signatures.

// Feature: Type aliases — `type Name = T`

// Syntax: `type Alias = ExistingType`

// When to use: give a semantic name to an existing type without

// creating a new type. Aliases are TRANSPARENT — `type Meters = float`

// accepts any `float`. For an opaque type, see 12-newtypes.


// Numeric domains.

type Meters = float
type Seconds = float
type Velocity = float

fn speed(distance: Meters, time: Seconds) -> Velocity {
  return distance / time
}

let d: Meters = 100.0
let t: Seconds = 9.58
print(speed(d, t))  // ~10.44


// Alias for collections.

type IdList = [int]

fn mark_processed(ids: IdList) {
  print("processing {ids.len()} ids")
}

mark_processed([1, 2, 3])

// Aliases do NOT prevent mixing compatible types — they are just names.

let mixed: Meters = 9.58  // ok even though it conceptually is "time"

print(mixed)
// The typechecker does not distinguish Meters from Seconds.

// If you want that distinction, use `newtype` (next file).