Type Checking

Array

In general JSON, Arrays can contain various values.

// In JSON, this is OK.
[1, 3.14, ["cumin"]]

But this is very strange and buggy. In cumin, Arrays should contain values with same type.

// In cumin, this is NG.
[1, 3.14, ["cumin"]]

This occurs and error

Error: Cannot infer type of Array([Nat(1), Float(3.14), Array(String, [Str("cumin")])]); Hint: Array cannot contain values with different types.

Any Type

Any is the top type.

let x: Any = 100;

This is ok. And don't worry. cuminc knows that x is Nat (because 100 is Nat). Any is convenient in some cases. This example is trivial, and it is equivalent to let statement without type annotation.

For example, Array<Any> is a type for Something Arrays.

let xs: Array<Any> = [
    1, 2, 3
];

In this example,

1. The elements are all Nat.
2. xs is a something array Array<_>.
3. These facts conclude that xs is Array<Nat>.

Because _ is an alias for Any, you can write

let xs: Array<_> = [
    1, 2, 3
];

It is unsafe that struct fields are declared as Any.

struct Data {
    data: Any,
}

Since data can be any values, followings are all valid.

let x = Data {
    data = 1,  // Nat
};

let y = Data {
    data = 3.14,  // Float
};

let z = Data {
    data = ["cumin"]  // Array<String>
};

And cuminc knows only that x, y and z are just Data, and ignores the type of data. So they have all same type!

Hack: Array with various data.

In cumin v0.9.7, This is ok.

struct Data {
    data: Any,
}

let x = Data {
    data = 1,  // Nat
};

let y = Data {
    data = 3.14,  // Float
};

let z = Data {
    data = ["cumin"]  // Array<String>
};

[x, y, z]

Because the last data is just Array<Data>. NOTE: No warrantry to support this hack.