Add Exhaustive Pattern Matching Of List Variants as a reason til

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

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+# 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)