when pattern matching comes to a surprise

Adrian Veith

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Sep 20, 2017, 8:19:07 AM9/20/17

to Haxe

Hi List,

just stumbled upon a gotcha with pattern matching, which I wanted to share. We have a new coworker, who starts working with Haxe. He wrote some code, like the snippet below. In the case of the "h" he omitted the quotes and to my surprise the code did compile. It took me a while to grasp that pattern matching did its magic here:


class Test {
    static function main() {
        var input = "x";
        var answer = "";
        switch (input) {
           case "l":
                answer = "input was l";
           case h:
                answer = "input was h";
           default:
                answer = "dont know";
        }
        trace(answer); // "input was h"
    }
}

this can lead to nasty errors. Imagine this:

class Test {
    static inline var keyUP = "u";
    static inline var keyDOWN = "d";
    
    static function main() {
        var input = "d";
        var answer = "";
        switch (input) {
           case keyUPS:
                answer = "input was up";
           case keyDOWN:
                answer = "input was down";
           default:
                answer = "dont know";
        }
        trace(answer); // "input was up"
    }
}

now I know, that it is they way it is. Constants should be UPPERCASE !

Simon Krajewski

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Sep 20, 2017, 8:28:07 AM9/20/17

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Am 20.09.2017 um 14:19 schrieb Adrian Veith:

Hi List,

just stumbled upon a gotcha with pattern matching, which I wanted to share. We have a new coworker, who starts working with Haxe. He wrote some code, like the snippet below. In the case of the "h" he omitted the quotes and to my surprise the code did compile. It took me a while to grasp that pattern matching did its magic here:

class Test { static function main() { var input = "x"; var answer = ""; switch (input) { case "l": answer = "input was l"; case h: answer = "input was h"; default: answer = "dont know"; } trace(answer); // "input was h" } }

This should give you a warning about an unused case though.

Simon

Adrian Veith

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Sep 20, 2017, 9:29:33 AM9/20/17

to Haxe

Yeah, maybe we need a warning to read the warnings ;)

Benjamin Dasnois

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Sep 20, 2017, 11:03:21 AM9/20/17

to haxe...@googlegroups.com

It's a feature, not a bug.

Sorry, had to write it. Oh and by the way : I'm back! ;D

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