std::data::opt

opt 'a represents an optional value. It has two variants:

pub enum opt 'a = nil | just 'a

The prelude re-exports opt, just, and nil, so user code normally writes them unqualified.

Constructing

nil
just 42
just "hello"

Pattern-matching

my maybe_text = just "hello"

case maybe_text:
    just text -> text.len
    nil       -> 0

The compiler does not check case expressions for exhaustiveness. A matching single arm is accepted:

case just 1:
    just x -> x

This expression returns 1 without a nil arm. Cover both variants or add a wildcard when the code needs to handle either value.

Common shapes

my maybe_text = just "notes"
my s = "21"

// Default value
case maybe_text:
    just text -> text
    nil       -> "(no file)"

// Chain through a fallible step
case s.to_int:
    just n  -> just (n * 2)
    nil     -> nil

For more combinators, convert the value to result in project code (see std::data::result) or define only the helpers the project needs.

Quick reference

Operation	Signature
nil	opt 'a
just(x)	'a -> opt 'a

See also

• std::data::result — when failure carries information
• Patterns → Enum patterns — variants in patterns
• Errors — typed failures expressed as effects