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retired
A standard library for the Gleam programming language
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src/gleam/result.gleam
import gleam/list
/// 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)
/// 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.
///
pub type Nil =
Nil
/// Checks whether the result is an Ok value.
///
/// ## Examples
///
/// > is_ok(Ok(1))
/// True
///
/// > is_ok(Error(Nil))
/// False
///
pub fn is_ok(result: Result(a, e)) -> Bool {
case result {
Error(_) -> False
Ok(_) -> True
}
}
/// Checks whether the result is an Error value.
///
/// ## Examples
///
/// > is_error(Ok(1))
/// False
///
/// > is_error(Error(Nil))
/// True
///
pub fn is_error(result: Result(a, e)) -> Bool {
case result {
Ok(_) -> False
Error(_) -> True
}
}
/// Updates 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)
///
pub fn map(over result: Result(a, e), with fun: fn(a) -> b) -> Result(b, e) {
case result {
Ok(x) -> Ok(fun(x))
Error(e) -> Error(e)
}
}
/// Updates 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)
///
pub fn map_error(
over result: Result(a, e),
with fun: fn(e) -> f,
) -> Result(a, f) {
case result {
Ok(x) -> Ok(x)
Error(error) -> Error(fun(error))
}
}
/// Merges a nested Result into a single layer.
///
/// ## Examples
///
/// > flatten(Ok(Ok(1)))
/// Ok(1)
///
/// > flatten(Ok(Error(""))
/// Error("")
///
/// > flatten(Error(Nil))
/// Error(Nil)
///
pub fn flatten(result: Result(Result(a, e), e)) -> Result(a, e) {
case result {
Ok(x) -> x
Error(error) -> Error(error)
}
}
/// Updates 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)
///
pub fn then(
result: Result(a, e),
apply fun: fn(a) -> Result(b, e),
) -> Result(b, e) {
case result {
Ok(x) -> fun(x)
Error(e) -> Error(e)
}
}
/// Extracts 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
///
pub fn unwrap(result: Result(a, e), or default: a) -> a {
case result {
Ok(v) -> v
Error(_) -> default
}
}
/// Extracts the Ok value from a result, evaluating the default function if the result
/// is an Error.
///
/// ## Examples
///
/// > lazy_unwrap(Ok(1), fn() { 0 })
/// 1
///
/// > lazy_unwrap(Error(""), fn() { 0 })
/// 0
///
pub fn lazy_unwrap(result: Result(a, e), or default: fn() -> a) -> a {
case result {
Ok(v) -> v
Error(_) -> default()
}
}
/// Transforms any error into Error(Nil)
///
/// ## Examples
///
/// > nil_error(Error(1))
/// Error(Nil)
///
/// > nil_error(Ok(1))
/// Ok(1)
///
pub fn nil_error(result: Result(a, e)) -> Result(a, Nil) {
map_error(result, fn(_) { Nil })
}
/// Returns the first value if it is Ok, otherwise return the second value.
///
/// ## Examples
///
/// > or(Ok(1), Ok(2))
/// Ok(1)
///
/// > or(Ok(1), Error("Error 2"))
/// Ok(1)
///
/// > or(Error("Error 1"), Ok(2))
/// Ok(2)
///
/// > or(Error("Error 1"), Error("Error 2"))
/// Error("Error 2")
///
pub fn or(first: Result(a, e), second: Result(a, e)) -> Result(a, e) {
case first {
Ok(_) -> first
Error(_) -> second
}
}
/// Returns the first value if it is Ok, otherwise evaluates the given function for a fallback value.
///
/// ## Examples
///
/// > or(Ok(1), Ok(2))
/// Ok(1)
///
/// > or(Ok(1), Error("Error 2"))
/// Ok(1)
///
/// > or(Error("Error 1"), Ok(2))
/// Ok(2)
///
/// > or(Error("Error 1"), Error("Error 2"))
/// Error("Error 2")
///
pub fn lazy_or(
first: Result(a, e),
second: fn() -> Result(a, e),
) -> Result(a, e) {
case first {
Ok(_) -> first
Error(_) -> second()
}
}
/// Combines a list of results into a single result.
/// If all elements in the list are Ok then returns an Ok holding the list of values.
/// If any element is Error then returns the first error.
///
/// ## Examples
/// > all([Ok(1), Ok(2)])
/// Ok([1, 2])
///
/// > all([Ok(1), Error("e")])
/// Error("e")
pub fn all(results: List(Result(a, e))) -> Result(List(a), e) {
list.try_map(results, fn(x) { x })
}
pub fn replace_error(result: Result(a, e1), error: e2) -> Result(a, e2) {
result
|> map_error(fn(_) { error })
}