Scope & mutation

Where names live, what functions can see, and which values are shared. Run a cell to see its output beside the expected one.

Functions read globals

A top-level scalar is visible inside every def.

Viv [1]

g = 5.0
def f() {
    return g + 1.0
}
show f()

Assignment in a function body creates a local variable; the global g keeps its value, 5.

Viv [2]

g = 5.0
def f() {
    g = 99.0
    return g
}
show f()
show g

A def that stores into an element of a global array changes the global array; no return is needed.

Viv [3]

tape = zeros(3)
def store(v) {
    tape[0] = v
    return 0.0
}
x = store(42.0)
show tape

b = a makes b refer to the same array; it does not copy the data. Changing b changes a.

Viv [4]

a = [1.0, 2.0]
b = a
b[0] = 99.0
show a

A function mutating its array parameter mutates the caller's array.

Viv [5]

def poke(v) {
    v[0] = 77.0
    return 0.0
}
a = [1.0, 2.0]
x = poke(a)
show a

Capture is by reference for globals: rebinding y changes what f computes.

Viv [6]

y = 2.0
f = fn(x) -> x * y
y = 10.0
show f(3.0)