Type Inference

Goals

• Keep inference local and predictable.
• Require explicit types on public APIs.

Inference never crosses module boundaries. Public signatures are always explicit.

Inference rules (v0.1)

• let x = 1; infers i32.
• Function parameters require explicit types.
• Function return types must be explicit.
• Use unit for functions that do not return a value.
• Lambda inference behavior is defined in docs/spec/lambdas-and-closures.md.
• for item in expr infers item from iterator Next() -> Option<T> item type.

Examples:

let count = 1; // i32

i32 add(i32 a, i32 b) { return a + b; }

i32 local_sum(i32 a, i32 b) { // private; return type explicit
    return a + b;
}

Decisions

• Generic function inference is planned for a future version (not in v0.1).

Example

T id<T>(T x) { return x; }

let a = id<i32>(5); // required in v0.1