Type Annotation

Typed Let

let statement may have type annotations.

let x: Int = 100;

cuminc evaluates this in the following steps:

1. eval 100
   • the type infered as Nat because it is a (non-negative) natural number.
2. natural cast
   • x is annotated as Int.
   • Nat can be casted to Int naturally.
   • get 100 as Int.
3. name it x

In the natural cast, cuminc doesn't coerce forcibly. For example, String to Int, Int to Nat. NOTE: If you need, as-cast coerce to other types.

The type annotation is optional. If it is omitted, the step 2 will be skipped.

let x = 100;

In this example, x is Nat.

Typed Struct

In structs, all fields should be type annotated.

struct S {
    x: Nat,
    y: Int,
    z: Array<String>,
}

When constructing struct values (applying), cuminc checks the types of applied values.

S {
    x = 1,
    y = -2,
    z = ["cumin"],
}