Variables

Bindings are immutable by default (=). Use := to declare a mutable binding and to reassign it.

x = 42                  ~ immutable bind (rebinding x with = is an error)
counter := 0            ~ mutable bind
counter := counter + 1  ~ reassign (also :=)

Reassigning requires the binding to be mutable: x := 5 on an immutable x is an error. Types are inferred but can be annotated: x :: Num = 42.

A top-level binding must be a constant or a function

A binding written outside any function is a global. A global’s initializer has to be a constant already: no Quilon code runs before ^, so there is nowhere to compute one. The value may be a Num, Bool or $ literal, or a function. Mutable (:=) globals follow the same rule, and a := global is writable from inside a function like any other.

limit = 10              ~ fine
enabled = true          ~ fine
scale = (n :: Num) => n * 3   ~ fine — a function value
counter := 4            ~ fine — and writable from a function

doubled = limit * 2     ~ error: has to be computed
greeting = "hi"         ~ error: a Text is built at runtime
sizes = [1, 2]          ~ error: an array is built at runtime
origin = { x = 0 }      ~ error: so is a record

A rejected binding reports what it is and how to fix it — move the work into the function that uses it. Anything computed is perfectly ordinary inside a function; the restriction is only about globals. (See examples/globals.qn and examples/global_computed.qn.)