# function: naming a reusable job

[Syntax and values](README.md)

```text
function add(a, b):
    return a + b

say add(2, 3)
```

Parameters are comma-separated identifiers. Empty parameter lists and trailing
commas are allowed. See [Section 11](../../../pliro-syntax.en.md#11-functions) for function semantics.

## Declaration and call

```pliro
# language: en
function factorial(number):
    if number <= 1:
        return 1
    return number * factorial(number - 1)

say factorial(5)
```

Functions have a fixed number of positional parameters. User-function arity is
checked when the callee can be resolved statically and again at runtime for a
dynamic call.

Pliro currently has no default, named, optional, variadic, typed, or destructured
parameters.

Arguments are passed as values. Lists and maps retain the collection-sharing
behavior described in [Section 9.6](../../../pliro-syntax.en.md#96-collection-sharing), so a function can
update an existing element of a collection supplied by its caller.

## Return value

`return expression` returns a value. Bare `return` and function fallthrough
return `none`:

```text
function announce(message):
    say message
    return
```

## Closures and nested functions

User functions close over their lexical environment:

```pliro
# language: en
function makeCounter(start):
    let value = start

    function next():
        set value = value + 1
        return value

    return next

let counter = makeCounter(10)
say counter()
say counter()
```

Functions are first-class runtime values: they can be stored, passed, returned,
and called. There is no anonymous-function or lambda syntax.

## Recursion limits

Direct recursion is supported. Runtimes enforce a bounded maximum user-function
call depth and produce a structured runtime error instead of overflowing the
host stack. Session runtimes make this bound configurable; the current generated
native runtime fixes it at 256.

## Related entries

[Syntax and values](README.md) · [Built-ins](../builtins/README.md) · [Programming guides](../guides/README.md)