Anonymous functions

Use def without a name to create a function value. Parameters and the return value need explicit types. Inside brackets, such as a call’s arguments, the body is one expression and its value is returned:

def apply(f: Callable[[i64], Result[i64, OverflowError[i64]]], value: i64) -> Result[i64, OverflowError[i64]]:
    f(value)

def main() -> Result[(), AllocError | IoError | OverflowError[i64]]:
    print(apply(def(x: i64) -> Result[i64, OverflowError[i64]]: Ok(x * 2), 4)?)?
    print(apply(
        def(x: i64) -> Result[i64, OverflowError[i64]]: Ok(x + 1 if x > 0 else 0),
        4,
    )?)?
    Ok(())

8
5

The body ends at the comma or closing bracket that ends the argument. Each function has type Callable[[i64], Result[i64, OverflowError[i64]]] and can be stored, returned, or passed to a callback parameter.

Outside brackets, the body is an indented block with the same rules as a named function:

def main() -> Result[(), IoError | OverflowError[i64]]:
    transform = def(value: i64) -> Result[i64, OverflowError[i64]]:
        adjusted = value * 2
        if adjusted > 10:
            return Ok(10)
        Ok(adjusted + 1)
    print(transform(3)?)?
    print(transform(8)?)?
    Ok(())

7
10

The position decides the form. A function with statements cannot be written inside brackets; give it an indented body and a name first, as transform has:

def apply(f: Callable[[i64], Result[i64, OverflowError[i64]]], value: i64) -> Result[i64, OverflowError[i64]]:
    f(value)
def main() -> ():
    apply(def(x: i64) -> Result[i64, OverflowError[i64]]: return Ok(x * 2), 4)

inside brackets an anonymous function body is one expression, and its value is returned; remove `return`

Parentheses are brackets, so a one-expression function can still be bound: double = (def(x: i64) -> Result[i64, OverflowError[i64]]: Ok(x * 2)). Python’s lambda is not available; the compiler names the def spelling instead.