Bindings & Mutation

This page covers how variables are declared, destructured, and reassigned in TypR.

Declaration with let

let x <- 42;
let y: int <- 5;
@pub let z <- "hello";             # public + testable
@testable let cache <- state(0);   # private, visible in build --test via M$.test_cache
@export let api <- fn(x: int): int { x };  # @pub + #' @export roxygen2

Both <- and = work as the assignment operator in let. However, a single = is never a binary infix operator in expressions — it is reserved for named fields (x = 1), top-level assignment, and default parameter values.

---

Tuple destructuring

let :{a, b, c} <- :{1, 2, 3};
let :{a, _, c} <- :{1, 2, 3};   # wildcard: ignore the element

Destructuring is desugared into a temporary variable + positional access via dot (__tuple_tmp__.1, .2, ...).

---

Reassignment & mutation

x <- 10;         # reassign an already-bound variable
x <- x + 1;

# "implicit mutation" sugar: x!; reassigns x to the result of the expression
x |> f() |> g()!;         # ≡  x <- x |> f() |> g();
obj.method()!;            # ≡  obj <- obj.method();

The expr!; form requires the head of the ./|> chain to be an assignable variable — 3!; is rejected at parse time.

---

Visibility annotations

Annotation	Effect
@pub	makes the binding public (visible outside the module)
@testable	private, but exposed as M$.test_name in build --test mode
@export	combines @pub with a #' @export roxygen2 tag