type-inference.md

dyvil	v0.31.0

Type Inference

Dyvil supports type inference for members of kind var, let, const and func.

const s = "a" // s: String
func charCount(s: String) = s.length // charCount(s: String) -> int 
var i = charCount(s) // i: int
let a = [s, i] // a: [String | int]

This also works for other types such as generic types, tuples or functions.

let list  = List(1, 2, 3)                // List<int>
let fun   = (s: String) => s.toUpperCase // String -> String
let tuple = (1, "a")                     // (int, String)

Local Type Inference

Unlike other functional programming languages like Haskell, the type inference in Dyvil is local. This means that type inference only flows in one direction:

func myMethod(i, j) = i + j // invalid, i and j require explicit type

An advanced functional programming language could infer the types of the parameters i and j: a numeric type that supports the + operation.

However, Dyvil requires you to explicitly declare the type of the method parameters:

func myMethod(i: int, j: int) = i + j

While the return type can be inferred to int, it is recommended for public API methods to explicitly declare their return type without relying on type inference.