Higher-order Functions

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# Higher-order Functions

_Functions that take and return other functions._

Functions can take and return other functions. This is how the collection builtins like `map`, `filter`, and `fold` are parameterised.

```wcl
adder = fn(x: i32) -> fn(i32) -> i32 fn(y: i32) -> i32 x + y
add3  = adder(3)
seven = add3(4)

doubled = map([1, 2, 3], fn(x: i64) -> i64 x * 2)
```

## Related

- [Function Literals](../references/concept_function_literals.md)

- [fn Items](../references/concept_fn_items.md)

- [Function Types](../references/concept_function_types.md)

- [Lists](../references/concept_lists.md)

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