A simple function returning Result might be defined and used like so: Pattern matching on Result s is clear and straightforward for simple cases, but Result comes with some convenience methods that make working with it more succinct.
A common problem with using return values to indicate errors is that it is easy to ignore the return value, thus failing to handle the error. These makes Result especially useful with functions that may encounter errors but don't otherwise return a useful value.
If you do write that in Rust, the compiler will give you a warning (by default, controlled by the unused_must_use lint). You might instead, if you don't want to handle the error, simply assert success with expect.
This will panic if the write fails, providing a marginally useful message indicating why: When writing code that calls many functions that return the Result type, the error handling can be tedious.
, hides some boilerplate of propagating errors up the call stack. Will result in the unwrapped success (Ok) value, unless the result is Erred, in which case Err is returned early from the enclosing function.
In Rust, errors can be classified into two major categories as shown in the table below. A program can retry the failed operation or specify an alternate course of action when it encounters a recoverable error.
NOTE The program prints end of the main event though file was not found. The standard library contains a couple of helper methods that both ends Result
You can use them to simplify error cases where you really do not expect things to fail. In case of success from a method, the “unwrap” function is used to extract the actual result.
If it INSERR or None instead, it raises a panic with the contents of the error displayed. Behaves like unwrap, except that it outputs a custom message before panicking in addition to the contents of the error.
The unwrap() function will panic and return a default error message as shown below The program can return a custom error message in case of a panic.
The only difference is that a custom error message can be displayed using expect. #pub coast FN is_ok (self) built Returns true if the result is Ok.
#pub coast FN is_err (self) built Returns true if the result is Erred. Converts self into an Option
Converts self into an Option
Pub FN map (self, op: F) Result where F: Nonce (T) U, Maps a Result
Pub FN map_or (self, default: U, f: F) U where F: Nonce (T) U, 1.41.0 Applies a function to the contained value (if Ok), or returns the provided default (if Err). Arguments passed to map_or are eagerly evaluated; if you are passing the result of a function call, it is recommended to use map_or_else, which is lazily evaluated.
Pub FN map_or_else (self, default: D, f: F) U where D: Nonce (E) U, F: Nonce (T) U, 1.41.0 Maps a Result
Pub FN map_err
Returns a mutable iterator over the possibly contained value. Calls op if the result is Ok, otherwise returns the Err value of self.
This function can be used for control flow based on Result values. Pub FN or
Arguments passed to or are eagerly evaluated; if you are passing the result of a function call, it is recommended to use or_else, which is lazily evaluated. Pub FN or_else
This function can be used for control flow based on result values. Pub FN unwrap_or (self, default: T) T Returns the contained Ok value or a provided default.
Arguments passed to unwrap_or are eagerly evaluated; if you are passing the result of a function call, it is recommended to use unwrap_or_else, which is lazily evaluated. Returns the contained Ok value or computes it from a closure.
Pub FN expect (self, msg: & STR) T 1.4.0 Returns the contained Ok value, consuming the self value. Panics if the value is an Err, with a panic message including the passed message, and the content of the Err.
Panics if the value is an Err, with a panic message provided by the Err's value. Panics if the value is an Ok, with a panic message including the passed message, and the content of the Ok.
Consumes the self argument then, if Ok, returns the contained value, otherwise if Err, returns the default value for that type. Converts a string to an integer, turning poorly-formed strings into 0 (the default value for integers).
Parse converts a string to any other type that implements Frost, returning an Err on error. Returns the contained Ok value, but never panics.
Unlike unwrap, this method is known to never panic on the result types it is implemented for. Therefore, it can be used instead of unwrap as a maintainability safeguard that will fail to compile if the error type of the Result is later changed to an error that can actually occur.
Flattening once only removes one level of nesting: Pub FN from (res: &StreamResult) Result
Here is another example that tries to subtract one from another list of integers, this time checking for underflow: Here is a variation on the previous example, showing that no further elements are taken from inter after the first Err.
Returns a consuming iterator over the possibly contained value. #FN max (self, other: Self) Self 1.21.0 Compares and returns the maximum of two values.
#FN min (self, other: Self) Self 1.21.0 Compares and returns the minimum of two values. #FN clamp (self, min: Self, max: Self) Self This is a nightly-only experimental API.
This method returns an ordering between self and other values if one exists. This method tests less than or equal to (for self and other) and is used by the operator.
Should not Err occur, the product of all elements is returned. Should not Err occur, the sum of all elements is returned.
This sums up every integer in a vector, rejecting the sum if a negative element is encountered: Is called to get the representation of the value as status code.
A return of Err(e) means that execution should branch to the innermost enclosing catch, or return from the function. Wrap an OK value to construct the composite result.