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Welcome to module 8, 

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<i>Useful methods on</i> 

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<i>result and option</i>. 

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In this module, 

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we're going to cover the most useful methods 

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the Standard Library has defined on 

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<code>Result</code> and <code>Option</code>. 

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We'll see how to test for which variant we have, 

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how to convert a <code>Result</code> to an <code>Option</code> 

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and vice versa, 

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how to specify a fallback value 

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to use in the error 

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and <code>None</code> cases, 

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and how to transform the <code>Ok</code> 

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or <code>Some</code> values further, 

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but leave error or <code>None</code> values as they 

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are. 

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Let's start with testing to see which variant 

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we have. 

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The names of these methods are different 

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for the two types. 

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<code>Result</code> has the methods <code>is_ok</code> 

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and <code>is_err</code> for testing 

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whether a value of type <code>Result</code> 

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is the <code>Ok</code> variant or the <code>Err</code> 

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variant. 

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<code>Option</code> has the methods <code>is_some</code> 

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and <code>is_none</code> that test whether 

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a value is the <code>Some</code> variant 

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or the <code>None</code> variant. 

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These methods are useful when you want 

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to know which variant you have, 

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but either don't want to or can't 

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test what the values within the variants 

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are. 

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These tests could be asserts 

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in automated tests 

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or in conditionals used for control flow. 

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These methods shouldn't be used in 

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combination 

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with <code>unwrap</code> - 

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the method that returns the inner value 

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of an <code>Ok</code> or <code>Some</code> 

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and panics otherwise. 

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An example of this antipattern 

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is checking if a value is <code>some</code> 

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and unwrapping it if so. 

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Yes, this ensures the <code>unwrap</code> 

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won't cause a panic, 

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but the code isn't very clear. 

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You'd have to read this code carefully to 

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notice that it won't panic. 

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The code is also doing a redundant check 

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of which variant the value is. 

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A better choice with regards to code readability 

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and runtime performance would be to 

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use <code>match</code>, <code>if let</code>, 

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or one of the methods we'll be covering in the 

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rest of the module. 

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Next, 

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let's talk about converting between <code>Result</code> 

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and <code>Option</code>. 

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To convert a <code>Result</code> into an <code>Option</code>, 

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you can use the <code>ok</code> method. 

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If you call <code>ok</code> on an 

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<code>Ok</code> value, you'll get 

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a <code>Some</code> variant holding the same 

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value that the <code>Ok</code> was holding. 

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If you call <code>ok</code> on an <code>Err</code> 

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value, 

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you'll get a <code>None</code> variant, 

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and the value inside the error will be 

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thrown away. 

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To convert an <code>Option</code> into a <code>Result</code>, 

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you can use the <code>ok_or</code> 

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method. 

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If you call <code>ok_or</code> 

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on a <code>Some</code> value,  

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you'll get an <code>Ok</code> variant holding the same 

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value that the <code>Some</code> was holding. 

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When we converted from <code>Result</code> to <code>Option</code>, 

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we lost the error information, 

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which means to convert from <code>Option</code> to <code>Result</code>, 

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we need to add information - 

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the error value that <code>None</code> should correspond 

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to. 

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That's what the argument to 

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<code>ok_or</code> is. 

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If the value is <code>None</code>, calling 

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<code>ok_or</code> will return the argument 

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inside an <code>Err</code> variant. 

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Next, let's talk about specifying 

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fallback values by using the 

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<code>unwrap_or</code> method. 

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If called on <code>Ok</code> 

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or <code>Some</code>, 

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<code>unwrap_or</code> extracts the value 

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from the variant, 

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and returns that 

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so you can use it. 

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If called on <code>Err</code> 

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or <code>None</code>, 

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<code>unwrap_or</code> returns the fallback 

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value specified as an argument. 

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For example, in the module where 

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we introduced <code>Result</code> and <code>Option</code>, 

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we tried to parse JSON that may 

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or may not be valid. 

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We used the <code>match</code> expression to return 

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a fallback <code>Person</code> value, 

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if there is an error. 

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Instead of the <code>match</code>, 

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we can use <code>unwrap_or</code> 

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and specify the fallback 

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value. 

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We can shorten this code further 

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using the 

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<code>unwrap_or_default</code> method. 

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This method doesn't take an argument 

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but always uses the type's 

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default value. 

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It makes use of the functionality defined 

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by the <code>Default</code> trait 

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and otherwise has the same behaviour as 

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<code>unwrap_or</code>. 

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So, if the type inside the <code>Ok</code> 

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derives or implements the <code>Default</code> trait, 

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you can use the 

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<code>unwrap_or_default</code> method. 

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Next, let's talk about a method 

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that lets us transform <code>Ok</code> 

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or <code>Some</code> values further. 

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This method is called <code>map</code>. 

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It's useful when you want to perform an 

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operation on the value  in 

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<code>Ok</code> or <code>Some</code>. 

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But leave <code>Err</code> 

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or <code>None</code> values as they are. 

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Here's a simplified example 

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from <code>crates.io</code> code. 

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We have a <code>Version</code> struct that may 

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or may not have the <code>id</code> of the 

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user who published that crate version 

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because we only recently started tracking 

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that information. 

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To get the <code>User</code> struct that goes 

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with the user ID in the version, 

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we've defined the <code>published_by</code> method 

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that has the return type <code>Option<User></code>.

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If the version's <code>published_by</code> attribute 

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is the <code>Some</code> variant, 

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meaning it has an ID, 

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we use <code>map</code> to look up the user 

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in the database. 

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If <code>published_by</code> is <code>None</code>, 

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the <code>map</code> does nothing 

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and passes the <code>None</code> value through. 

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We've provided simplified definitions 

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for the <code>User</code> struct 

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and the user <code>find</code> function 

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so that we can run this <code>main</code> function 

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that creates a version that has the 

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user <code>id</code> 

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and a version that doesn't. 

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When we run this code, 

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we see the result of calling <code>map</code> 

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is <code>Some(User</code> struct instance 

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if <code>published by</code> is <code>Some</code>, 

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and <code>None</code> if <code>published by</code> is <code>None</code>. 

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There are many more methods defined on 

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<code>Result</code> and <code>Option</code> than we can 

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cover in this video. 

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Lots are slight variations on the ones 

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we have touched on, 

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such as methods that take a closure to 

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apply, 

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rather than a value 

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or vice versa. 

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By learning about some of these methods, 

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you now have a good idea of what many 

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of them do. 

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Overall, if you find yourself writing 

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a <code>match</code> expression to handle a <code>Result</code> 

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or <code>Option</code>, 

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and the <code>match</code> feels like it's a common operation 

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many people would want to do,

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 check the documentation to see if there is a method 

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that does what you're doing by hand. 

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In this module, 

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we discussed a bunch of useful methods on 

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<code>Result</code> and <code>Option</code>. 

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We covered how to test which variant you 

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have, 

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with <code>is_ok</code>, 

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<code>is_err</code>, <code>is_some</code>, 

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and <code>is_none</code>. 

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We showed how to convert results to options 

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with <code>ok</code>, 

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and options for results 

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with <code>okay_or</code>. 

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We talked about specifying fallback values 

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with <code>unwrap_or</code> 

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and <code>unwrap_or_default</code>. 

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And we looked at transforming values within 

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<code>Ok</code> or <code>Some</code> 

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using the <code>map</code> method. 

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That's the end of the <i>Error Handling</i> unit. 

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Next unit, 

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we'll learn all about lifetimes.