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34 lines
1.4 KiB
Markdown
34 lines
1.4 KiB
Markdown
# Pattern Match On Exceptions
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[ReasonML](https://reasonml.github.io/) supports [powerful pattern
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matching](https://reasonml.github.io/docs/en/pattern-matching.html) through
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the `switch` statement. This even includes pattern matching against
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exceptions that may arise as a way of catching and handling those
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exceptions.
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```reason
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let values: list(int) = [1,3,5,7,9];
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let getValue = (list, index) => {
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switch (List.nth(values, index)) {
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| value => value
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| exception Failure("nth") => 0
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| exception Invalid_argument("List.nth") => 0
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};
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};
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getValue(values, 0); /* 1 */
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getValue(values, 1); /* 3 */
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getValue(values, 5); /* 0 */
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getValue(values, -1); /* 0 */
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```
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The [`List.nth` docs](https://reasonml.github.io/api/List.html) detail what
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happens in the two kinds of out of bounds scenarios that would raise an
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error -- `Failure` and `Invalid_argument`. You can pattern match against
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those by declaring the respective cases as exception instances and then
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returning the desired values in response.
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[source
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code](https://reasonml.github.io/en/try.html?reason=DYUwLgBAbghsCuIDOAuCwCWSwAoMDswBKCAXggG0BGAGgGYaBWGgdhoE4BdAbgChfQkAObgAanERkIOTNhoQCAExAAPEqQB8EAN68IEJAHcMYAMYALaQBksYAHSFzOWAmTylqoiV36APtAkQMi0XRD0If1VTEAAHMAwAe3wIADEYDAQAJxAcACJHXPUtAAZwyJVouMTkgEl8FwxFAH0YTKF4AFsQQjybbAcwc0LgiFL9AF8+Sf4AKSQ7YAShHBEwcVdnQKR5Yq8+OYWllbFAzddtiCo93gPF5dX1xDPEC8Zr26OH09C3CABaK5EbhAA)
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