Result<E, T> shows an error handling with the variants, Err<E>, representing error and containing an error value, and Ok<T>, representing success and containing a passing value.

Functions overview

Almost of all, you don't need to access the internal value with [0] or [1] to check whether a Result<E, T> is Err<E> or Ok<T>. Instead you can use the functions provided by this module.

Constructors

ok and err functions construct the Result from value to contain. They have individual type Ok<T> and Err<E>, respectively. So sometimes you need to upcast into Result<E, T>.

Querying the variant

isOk and isErr functions return true if the Result is Ok or Err, respectively. These provides the type assertion on TypeScript.

Extracting contained values

These functions extract the contained value in a Result<E, T> when it is the Ok variant. If it is Err:

• unwrap throws an Error with a generic message
• unwrapOr returns the provided the default value
• unwrapOrElse returns the result of evaluating the provided function

And this function extracts the contained error value E in a Result<E, T>. If it is Ok:

• unwrapErr throws an Error with a generic message

Transforming contained values

These functions transform Result to Option:

• optionOk transforms Result<E, T> into Option<T>, mapping Ok<T> to Some<T> and Err<E> to None
• optionErr transforms Result<E, T> into Option<E>, mapping Err<E> to Some<E> and Ok<T> to None
• resOptToOptRes transposes a Result of an Option into an Option of a Result

This function transforms the contained value of the Ok variant:

• map transforms Result<E, T> into Result<E, U> by applying the provided function (t: T) => U to the contained value of Ok and leaving Err values unchanged
• product transforms Result<E, T> and Result<E, U> into a new Result<E, [T, U]> and leaving Err values occurred at first

This function transforms the contained value of the Err variant:

• mapErr transforms Result<E, T> into Result<F, T> by applying the provided function to the contained value of Err and leaving Ok values unchanged

These functions transform a Result<E, T> into a value of possibly different type U:

• mapOr applies the provided function to the contained value of Ok, or returns the provided default value if the Result is Err
• mapOrElse applies the provided function to the contained value of Ok, or applies the provided default fallback function to the contained value of Err

This functions transform both of the contained value at same time.

• biMap applies the two provided functions to the each contained value
• mergeOkErr merges the contained values of Result<T, T> into T

Boolean operators

These functions treat the Result as a boolean value, where Ok acts like true and Err acts like false. There are two categories of these methods: ones that take a Result as input, and ones that take a function as input (to be lazily evaluated).

The and and or methods take another Result as input, and produce a Result as output. The and method can produce a Result<E, U> value having a different inner type U than Result<E, T>. The or method can produce a Result<F, T> value having a different error type F than Result<E, T>. Note that the order of parameters are reversed because of convenience of partial applying.

function	first parameter	second parameter (as self)	output
and	ignored	err(e)	err(e)
and	err(d)	ok(x)	err(d)
and	ok(y)	ok(x)	ok(y)
or	err(d)	err(e)	err(d)
or	ok(y)	err(e)	ok(y)
or	ignored	ok(x)	ok(x)

The andThen and orElse functions take a function as input, and only evaluate the function when they need to produce a new value. The andThen function can produce a Result<E, T> value having a different inner type U than Result<E, T>. The orElse function can produce a Result<F, T> value having a different error type F than Result<E, T>.

function	first parameter	second parameter (as self)	output
andThen	ignored	err(e)	err(e)
andThen	f	ok(x)	f(x)
orElse	ignored	ok(x)	ok(x)
orElse	f	err(e)	f(e)