Today I Learned

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Pattern Matching On Exceptions

ReasonML supports powerful pattern matching through the switch statement. This even includes pattern matching against exceptions that may arise as a way of catching and handling those exceptions.

let values: list(int) = [1,3,5,7,9];

let getValue = (list, index) => {
  switch (List.nth(values, index)) {
  | value => value
  | exception Failure("nth") => 0
  | exception Invalid_argument("List.nth") => 0
  };
};

getValue(values, 0); /* 1 */
getValue(values, 1); /* 3 */
getValue(values, 5); /* 0 */
getValue(values, -1); /* 0 */

The List.nth docs detail what happens in the two kinds of out of bounds scenarios that would raise an error — Failure and Invalid_argument. You can pattern match against those by declaring the respective cases as exception instances and then returning the desired values in response.

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