Pattern Matching using switch in Reason

Nik Graf
InstructorNik Graf

Share this video with your friends

Send Tweet
Published 4 years ago
Updated 3 years ago

Pattern matching is probably one of the best features of the language. It comes down to two points:

  • check for a certain structure of a value
  • extract parts of that value

Since it's such an integral part of the language it comes with a lot of useful tricks like piping multiple patterns into one case. In this lesson we will explore all of them.

Instructor: [00:00] We have touched pattern matching in previous lessons. For example, simple leveraging its simplest form matching for structural equality. Because of structural equality, it also works with other data types like tuples, lists, variants, or records. We can pattern match on a to do tuple that has a description string and a check Boolean.

[00:41] As an alternative, we can use an underscore to match all previous unmatched values for this type. If you only care about the task being checked, we can do the following. A side effect of this change, we made our example exhaustive. Using a name instead of an underscore allows us to extract parts of the tuple.

[01:05] We can use text in both cases, but they don't have to be the same name. Of course, this works with any kind of type. For example, if we only care about the text, we could match the text and ignore the checked value.

[01:27] Next up, we explore pattern matching for other data structures. With lists, we can match on an exact list. Of course, an underscore can be used, as well. What's great about pattern matching with lists is that we can use the spread operator to extract the head and tail.

[01:49] Here, we still use the underscore. Once we leave it out, the compiler suggests to us an example we are missing. To how we match, the only case left to cover is an empty list. This allows us to be more explicit without writing a lot more code. In fact, this is the reason why destructuring lists outside of a switch expression is not recommended. The compiler will warn us since an empty list could lead to a run time error.

[02:24] The same goes for the destructuring of arrays. When pattern matching arrays, we can only match arrays of a specific length. The values of the array can be matched using structural equality. We can use an underscore or we extract them using a name.

[02:54] What about records? We, again, use a to do example, but this time in the form of a record. We first declare the type and then bind the to do to my to do. As you probably already have guessed, we can match the exact values. We are going to skip that though and move on to an example where we extract the text using the name description.

[03:32] By the way, we can use panning here. While explained in previous lessons, let's do a quick recap on pattern matching variants. We define a variant of type item with the constructor's note and to do. Using pattern matching, we can check for the constructors and extract parts of them.

[04:04] This is what pattern matching comes down to. With all the examples we've used until now, with each pattern, we can do two things at the same time. We can check what structure a value has and extract parts of the value.

[04:17] I hope by now you have a pretty good idea on how to leverage pattern matching. Since there are a couple more things that make our lives easier, we move on. Until now, we learned when we want to match multiple items and return the same result, we can do this.

[04:40] That's a lot of repetition. What we can do instead is using the pipe character to make sure multiple patterns result in the same case. The best part though, this works with any arbiter in nesting. As an example, we use a request with the possible state success and error. An error contains the error code.

[05:08] Using the pipe character, we can match multiple error codes when pattern matching for error. Sometimes though even that is not convenient or concise enough. Using the when keyword, we can even use custom logic as part of the pattern.

[05:31] For example, there are 12 documented HTTP server error codes. A function checking for the whole range would come in quite handy.

[05:44] Using when, we can apply the function like an if condition. Be aware by doing so, we lose the compiler's ability to check for exhaustiveness. If we remove the last entry, the compiler will warn us. It will also mention that this case might be already covered by the guarded clause. In general if possible, optimize your switch expressions to be exhaustive, especially when using variants. Meaning, try to avoid the when keyword, as well as the fall through case.

[06:14] Let me elaborate a bit on this. Here, we have the constructor slowly success and error. Now, think about having a switch expression in your code base that looks like this. Works as expected. Three months later, the business requirements change and we want the user to manually trigger fetching the request. This means we are extending our variant request with yet another constructor, not requested.

[06:48] If you rerun the existing switch expression, we notice that the compiler doesn't warn us about the new constructor. It's clearly a bug that the UI renders an error for the constructor not requested. Ideally, we wouldn't miss that. What would be even better is if the program wouldn't even compile if we forgot this new case. If we declare every case explicitly, the compiler will warn us. To make sure the compiler compiles and fixes the bug, we extend the switch expression.

[07:23] That's why it's recommended to avoid the fall through case and rather opting for explicitness. When matching for values, we can give them names using the S keyboard. This also works with nested structures. Here, we have a nested tuple. In the pattern we name the nested tuple number pair.

[07:49] Last but not least, there's one special case of pattern matching, ternary conditional. Reason's ternary is just syntax sugar for a Boolean switch.

Artem Korchunov
Artem Korchunov
~ 3 years ago

Hi Nik, you have missed ed ending for Not request there

Reason #
type request = 
  | Loading 
  | Success(string)
  | Error(int);
  | NotRequest;
type request = Loading | Success(string) | Error(int) | NotRequested;
Nik Graf
Nik Grafinstructor
~ 3 years ago

Good catch, thx Artem. Will notify the Egghead team to fix it.