Add Exhaustive Pattern Matching Of List Variants as a reason til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_612 TILs and counting..._
+_613 TILs and counting..._
 
 ---
 
@@ -440,6 +440,7 @@ _612 TILs and counting..._
 
 ### ReasonML
 
+- [Exhaustive Pattern Matching Of List Variants](reason/exhaustive-pattern-matching-of-list-variants.md)
 - [Pattern Match On Exceptions](reason/pattern-match-on-exceptions.md)
 - [String Interpolation With Integers And Sprintf](reason/string-interpolation-with-integers-and-sprintf.md)
 

reason/exhaustive-pattern-matching-of-list-variants.md

@@ -0,0 +1,32 @@
+# Exhaustive Pattern Matching Of List Variants
+
+ReasonML's `switch` expression allows for powerful pattern matching. When
+using `switch` to pattern match against a list, the compiler will be sure to
+warn you if you haven't accounted for all variants of a list.
+
+```reason
+let getFirst = (numList: list(int)): int => {
+  switch(numList) {
+  | [first, ...rest] => first
+  };
+};
+```
+
+> this pattern-matching is not exhaustive. Here is an example of a value
+> that is not matched: []
+
+The compiler knows that a list can either be 1) empty (`[]`) or 2) contain
+at least one value and another list (`[a, ...rest]`). To ensure all variants
+are accounted for, we can include the `[]` case in our switch.
+
+```reason
+let getFirst = (numList: list(int)): int => {
+  switch(numList) {
+  | [] => -1
+  | [first, ...rest] => first
+  };
+};
+```
+
+[source
+code](https://reasonml.github.io/en/try.html?reason=DYUwLgBAdgrgtgIxAJwM4C4LAJarACmyjAEoIBeCAbQDYAaABjvvoEYBdAbgChvRIA5uABi2NJEr5YcADK4wmHHkLESJTEQkA+CAG9uECKgDu2MAGMAFlPhy8ZfYYA+1AGZi8dCADpfyEHjsFDru4gYQAL48UbwAUqjewAD2AvhCYKLiNogoqGqcQA)