proposal for function refinement typechecking

[hiun]

Motivation

Functions in JavaScript is flexible, indeed it does not enforce parameter and return type.
In order to ensure correctness of program, typechecking for function refinement is needed.

Typechecking Rules

The rule for checking type validity for function refinement is simple, output(return) type of function must be covariant(subtyped) for input(parameter) of succeeding function. In other words, input(parameter) of function should be contravariant for its argument(returned data from preceding function)

Type Lists

Below table shows list of supported types

Name	Notation
Boolean Type	boolean
Number Type	number
String Type	string
Array Type	array
Any Type	any
Null Type	null
Undefined Type	undefined
Intersection Type (traits)	Object.assign(typeA, typeB)
Union Type (priority type)	typeA|typeB|typeC
Optional Type	?typeA

Examples