Docs / Standard Library / Option

Option

The Option module provides utilities for working with optional values. Option types represent values that may or may not exist, providing a safer alternative to null values.

Constructors

Options are created using the Some and None constructors, which are pre-registered in Kit and available without import.

Constructors

Some
a -> Option a
Wraps a value in a Some, indicating the value exists. 
value = Some 42
name = Some "Alice"
None
Option a
Represents the absence of a value. 
empty = None

Predicates

Option.is-some?
Option a -> Bool
Returns true if the option contains a value. 
println (Option.is-some? (Some 42))
# => true

println (Option.is-some? None)
# => false
Option.is-none?
Option a -> Bool
Returns true if the option is empty. 
println (Option.is-none? None)
# => true

println (Option.is-none? (Some 42))
# => false
is-some?
a -> Bool
Bare function that checks if a value is a Some variant. 
println (is-some? (Some "hello"))
# => true
is-none?
a -> Bool
Bare function that checks if a value is None. 
println (is-none? None)
# => true

Transforming

Option.map
(a -> b) -> Option a -> Option b
Applies a function to the value inside a Some, or returns None if empty. 
double = fn(x) => x * 2

result = Option.map double (Some 21)
# => Some(42)

empty = Option.map double None
# => None
Option.and-then
(a -> Option b) -> Option a -> Option b
Chains optional operations. If the input is Some, applies the function which may return None. Also known as "flatMap" or "bind" in other languages. 
parse-positive = fn(s) =>
    match String.to-int s
        | Some n if n > 0 -> Some n
        | _ -> None

result = Some "42" |> Option.and-then parse-positive
# => Some(42)

invalid = Some "-5" |> Option.and-then parse-positive
# => None

Unwrapping

Option.unwrap
Option a -> a
Returns the value inside a Some. Raises an error if None. Prefer Option.unwrap-or or pattern matching when possible. 
value = Option.unwrap (Some 42)
# => 42

# This will raise an error:
# Option.unwrap None
Option.unwrap-or
a -> Option a -> a
Returns the value inside a Some, or the provided default if None. 
value = Option.unwrap-or 0 (Some 42)
# => 42

default = Option.unwrap-or 0 None
# => 0

Pattern Matching with Options

Options are typically handled using pattern matching:

describe = fn(opt) =>
    match opt
        | Some value -> "Got: ${value}"
        | None -> "Nothing"

println (describe (Some 42))
# => "Got: 42"

println (describe None)
# => "Nothing"