2. Functions and the suspicious little plus

def square(+x: U32) -> U32:
  (x * x : U32)

def main() -> U32:
  square(7)

Read the signature aloud: “square receives a reusable U32 named x and produces a U32.” A call uses parentheses: square(7).

Why +x, rather than x? Bend variables are affine by default: they may be used at most once. We use x twice in multiplication, so we explicitly request reuse. U32 supports copying; chapter 6 explains how types tell us that.

The + in a binder is not addition, and it is not a sign on the number. It is permission to use that binding more than once.

Haskell companion:
square :: Word32 -> Word32
square x = x * x
Ordinary Haskell lets you use x zero, one, or many times without an annotation. Bend makes that resource choice explicit. Haskell's optional LinearTypes extension is related territory, but linear “exactly once” and Bend's affine “at most once” are not identical rules.

A local binding looks like this:

def main() -> U32:
  answer = square(7)
  (answer + 1 : U32)

Think of answer = ... as a let, not an update to a global mutable variable. Its final expression gives the result.

Try it: remove + from square and read the compiler's complaint. Restore it afterwards. Then define a cube function; how often does its argument appear?