Checked exceptions (original) (raw)

Remi Forax forax at univ-mlv.fr
Thu Oct 17 19:19:08 UTC 2019

----- Mail original -----

De: "Brian Goetz" <brian.goetz at oracle.com> À: "Thomas May" <tmay at clearwateranalytics.com>, "jdk-dev" <jdk-dev at openjdk.java.net> Envoyé: Jeudi 17 Octobre 2019 18:17:33 Objet: Re: Checked exceptions

The example that comes to mind is Double#parseDouble. It's annoying to have to deal with NumberFormatException, the API would jive better if instead it returned an Optional. But I realize that isn't possible without a lot of breakage, so what about instead a optionalParseDouble that returns a Optional?

Stepping back from the exceptions question for a minute, the way I would prefer to address problems like this is via pattern matching. If we expose a pattern whose target is a string, and which produces a binding variable of type double if the string is the string representation of a double, then either the pattern matches and produces a valid double, or it doesn't match.  The advantage of this over the Optional approach is that is nestable with other patterns, so if we have a pattern that produces a String, we can compose it (via nesting) with the double-parsing pattern. Once we have pattern matching, it will make sense to expose all sorts of parsing patterns, which work similarly to the Optional-bearing version of your method, but compose better.

It should compose as well because it's the extractor pattern which powers the pattern matching in Scala [1] (it's also called the railway switch pattern [2] by Scott Wlaschin).

interface Matcher { Optional match(String text);

default Matcher<T> or(Matcher<? extends T> matcher) {
  return text -> match(text).or(() -> matcher.match(text));
}

static Optional<Integer> parseDouble(String text) {
  return Optional.of(new Scanner(text)).filter(Scanner::hasNextDouble).map(Scanner::nextDouble);
}

}

Matcher parseDouble = Matcher::parseDouble; Matcher fortyTwo = __ -> Optional.of(42.0); Matcher matcher = parseInt.or(fortyTwo);

Rémi

[1] http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.640.9064&rep=rep1&type=pdf [2] https://fsharpforfunandprofit.com/rop/

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