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]