gleam/result
Types
Nil
Nil is a type used to represent the absence of something, similar to null or undefined in other languages.
Unlike some other languages values cannot be implicitly nil, value that may
be absent is typically represented using Result(TheType, Nil). This is
such a common type that offer the Option(TheType) alias.
pub type Nil = Nil
Option
A value that is either there or not there.
Some other languages have a dedicated Option type that is not related to Result for this, however this tends to have all the same functions as Result so in Gleam we combine the two.
pub type Option(value) = Result(value, Nil)
Result
Result represents the result of something that may succeed or not.
Ok means it was successful, Error means it was not successful.
pub type Result(success, error) = Result(success, error)
Functions
flatten
pub fn flatten(result: Result(Result(a, b), b)) -> Result(a, b)
Merge a nested Result into a single layer.
Examples
> flatten(Ok(Ok(1)))
Ok(1)
> flatten(Ok(Error(""))
Error("")
> flatten(Error(Nil))
Error(Nil)
is_error
pub fn is_error(result: Result(a, b)) -> Bool
Check whether the result is an Error value.
Examples
> is_error(Ok(1))
False
> is_error(Error(Nil))
True
is_ok
pub fn is_ok(result: Result(a, b)) -> Bool
Check whether the result is an Ok value.
Examples
> is_ok(Ok(1))
True
> is_ok(Error(Nil))
False
map
pub fn map( over result: Result(a, b), with fun: fn(a) -> c, ) -> Result(a, b)
Update a value held within the Ok of a result by calling a given function on it.
If the result is an Error rather than OK the function is not called and the result stays the same.
Examples
> map(over: Ok(1), with: fn(x) { x + 1 })
Ok(2)
> map(over: Error(1), with: fn(x) { x + 1 })
Error(1)
map_error
pub fn map_error( over result: Result(a, b), with fun: fn(b) -> c, ) -> Result(a, b)
Update a value held within the Error of a result by calling a given function on it.
If the result is Ok rather than Error the function is not called and the result stays the same.
Examples
> map_error(over: Error(1), with: fn(x) { x + 1 })
Error(2)
> map_error(over: Ok(1), with: fn(x) { x + 1 })
Ok(1)
none
pub fn none() -> Result(a, Nil)
Another way of writing Error(Nil).
Examples
> none()
Error(Nil)
then
pub fn then( result: Result(a, b), apply fun: fn(a) -> Result(c, b), ) -> Result(a, b)
Update a value held within the Ok of a result by calling a given function on it, where the given function also returns a result. The two results are then merged together into one result.
If the result is an Error rather than OK the function is not called and the result stays the same.
This function is the equivalent of calling map followed by flatten, and
it is useful for chaining together multiple functions that may fail.
Examples
> then(Ok(1), fn(x) { Ok(x + 1) })
Ok(2)
> then(Ok(1), fn(x) { Ok(tuple("a", x)) })
Ok(tuple("a", 1))
> then(Ok(1), fn(x) { Error("Oh no") })
Error("Oh no")
> then(Error(Nil), fn(x) { Ok(x + 1) })
Error(Nil)
unwrap
pub fn unwrap(result: Result(a, b), or default: a) -> a
Extract the Ok value from a result, returning a default value if the result is an Error.
Examples
> unwrap(Ok(1), 0)
1
> unwrap(Error(""), 0)
0