gleam/option

Types

Option

Option represents a value that may be present or not. Some means the value is present, None means the value is not.

This is Gleam’s alternative to having a value that could be Null, as is possible in some other languages.

pub type Option(a) {
  Some(a)
  None
}

Constructors

```
Some(a)
```
```
None
```

Functions

all

</>

pub fn all(list: List(Option(a))) -> Option(List(a))

Combines a list of Options into a single Option. If all elements in the list are Some then returns a Some holding the list of values. If any element is None then returnsNone.

Examples

all([Some(1), Some(2)])
// -> Some([1, 2])

all([Some(1), None])
// -> None

flatten

</>

pub fn flatten(option: Option(Option(a))) -> Option(a)

Merges a nested Option into a single layer.

Examples

flatten(Some(Some(1)))
// -> Some(1)

flatten(Some(None))
// -> None

flatten(None)
// -> None

from_result

</>

pub fn from_result(result: Result(a, b)) -> Option(a)

Converts a Result type to an Option type.

Examples

from_result(Ok(1))
// -> Some(1)

from_result(Error("some_error"))
// -> None

is_none

</>

pub fn is_none(option: Option(a)) -> Bool

Checks whether the Option is a None value.

Examples

is_none(Some(1))
// -> False

is_none(None)
// -> True

is_some

</>

pub fn is_some(option: Option(a)) -> Bool

Checks whether the Option is a Some value.

Examples

is_some(Some(1))
// -> True

is_some(None)
// -> False

lazy_or

</>

pub fn lazy_or(
  first: Option(a),
  second: fn() -> Option(a),
) -> Option(a)

Returns the first value if it is Some, otherwise evaluates the given function for a fallback value.

Examples

lazy_or(Some(1), fn() { Some(2) })
// -> Some(1)

lazy_or(Some(1), fn() { None })
// -> Some(1)

lazy_or(None, fn() { Some(2) })
// -> Some(2)

lazy_or(None, fn() { None })
// -> None

lazy_unwrap

</>

pub fn lazy_unwrap(option: Option(a), or default: fn() -> a) -> a

Extracts the value from an Option, evaluating the default function if the option is None.

Examples

lazy_unwrap(Some(1), fn() { 0 })
// -> 1

lazy_unwrap(None, fn() { 0 })
// -> 0

map

</>

pub fn map(
  over option: Option(a),
  with fun: fn(a) -> b,
) -> Option(b)

Updates a value held within the Some of an Option by calling a given function on it.

If the Option is a None rather than Some, the function is not called and the Option stays the same.

Examples

map(over: Some(1), with: fn(x) { x + 1 })
// -> Some(2)

map(over: None, with: fn(x) { x + 1 })
// -> None

or

</>

pub fn or(first: Option(a), second: Option(a)) -> Option(a)

Returns the first value if it is Some, otherwise returns the second value.

Examples

or(Some(1), Some(2))
// -> Some(1)

or(Some(1), None)
// -> Some(1)

or(None, Some(2))
// -> Some(2)

or(None, None)
// -> None

then

</>

pub fn then(
  option: Option(a),
  apply fun: fn(a) -> Option(b),
) -> Option(b)

Updates a value held within the Some of an Option by calling a given function on it, where the given function also returns an Option. The two options are then merged together into one Option.

If the Option is a None rather than Some the function is not called and the option stays the same.

This function is the equivalent of calling map followed by flatten, and it is useful for chaining together multiple functions that return Option.

Examples

then(Some(1), fn(x) { Some(x + 1) })
// -> Some(2)

then(Some(1), fn(x) { Some(#("a", x)) })
// -> Some(#("a", 1))

then(Some(1), fn(_) { None })
// -> None

then(None, fn(x) { Some(x + 1) })
// -> None

to_result

</>

pub fn to_result(option: Option(a), e: b) -> Result(a, b)

Converts an Option type to a Result type.

Examples

to_result(Some(1), "some_error")
// -> Ok(1)

to_result(None, "some_error")
// -> Error("some_error")

unwrap

</>

pub fn unwrap(option: Option(a), or default: a) -> a

Extracts the value from an Option, returning a default value if there is none.

Examples

unwrap(Some(1), 0)
// -> 1

unwrap(None, 0)
// -> 0

values

</>

pub fn values(options: List(Option(a))) -> List(a)

Given a list of Options, returns only the values inside Some.

Examples

values([Some(1), None, Some(3)])
// -> [1, 3]