prelude.Result

enum Result

A value, or an explanation of why there isn't one.

Failure is an ordinary value: no exceptions, so a function that can fail says so in its type.

let Result.is_ok (r: Result[T, E]): Bool

Whether this succeeded.

let Result.is_err (r: Result[T, E]): Bool

Whether this failed.

let Result.unwrap_or (r: Result[T, E]) (fallback: T): T

The value, or fallback on failure — DISCARDING the error. Reach for unwrap_or_else when the error should shape the answer.

let Result.unwrap_or_else (r: Result[T, E]) (f: (E) -> T): T

The value, or the result of calling f with the error. The recovery that gets to see what went wrong.

let Result.map (r: Result[T, E]) (f: (T) -> U): Result[U, E]

Transforms the value on success, leaving an error untouched.

let Result.map_err (r: Result[T, E]) (f: (E) -> F): Result[T, F]

Transforms the ERROR, leaving a success untouched. For adding context to a failure on its way up.

let Result.and_then (r: Result[T, E]) (f: (T) -> Result[U, E]): Result[U, E]

Chains an operation that may itself fail, stopping at the first error. The flat version of map over a fallible step.