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

flatten

</>

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

Merge 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

Check 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

Check whether the option is a Some value.

Examples

> is_some(Some(1))
True

> is_some(None)
False

map

</>

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

Update 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)

Return the first value if it is Some, otherwise return 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)

Update 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 Options.

Examples

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

> then(Some(1), fn(x) { Some(tuple("a", x)) })
Some(tuple("a", 1))

> then(Some(1), fn(x) { 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

Extract the value from an option, returning a default value if there is none.

Examples

> unwrap(Some(1), 0)
1

> unwrap(None, 0)
0