Random D geekout

[nhk]

Please bear with my ignorance I'm new to D, but why is that any
better compared to a simple

switch(key){
default: throw Exception("Invalid attribute '%s'".format(key));
case "name": d.name = value;
                break;
...
...
}



On Friday, 20 April 2012 at 04:05:43 UTC, H. S. Teoh wrote:
> I'm writing some code that does some very simplistic parsing, 
> and I'm
> just totally geeking out on how awesome D is for writing such 
> code:
>
> 	import std.conv;
> 	import std.regex;
> 	import std.stdio;
>
> 	struct Data {
> 		string name;
> 		string phone;
> 		int age;
> 		... // a whole bunch of other stuff
> 	}
>
> 	void main() {
> 		Data d;
> 		foreach (line; stdin.byLine()) {
> 			auto m = match(line, "(\w+)\s+(\w+)");
> 			if (!m) continue;
>
> 			auto key = m.captures[1];
> 			auto value = m.captures[2];
>
> 			alias void delegate(string key, string value) attrDg;
> 			attrDg[string] dgs = [
> 				"name": delegate(string key, string value) {
> 					d.name = value;
> 				},
> 				"phone": delegate(string key, string value) {
> 					d.phone = value;
> 				},
> 				"age": delegate(string key, string value) {
> 					d.age = to!int(value);
> 				},
> 				...	// whole bunch of other stuff to
> 					// parse different attributes
> 			];
> 			attrDg errordg = delegate(string key, string value) {
> 				throw Exception("Invalid attribute '%s'"
> 					.format(key));
> 			};
>
> 			// This is pure awesomeness:
> 			dgs.get(key.idup, errordg)(key.idup, value.idup);
> 		}
> 		// ... do something with Data
> 	}
>
> Basically, I use std.regex to extract keywords from the input, 
> then use
> an AA to map keywords to code that implement said keyword.  
> That AA of
> delegates is just pure awesomeness. AA.get's default value 
> parameter
> lets you process keywords and handle errors with a single AA 
> lookup.  I
> mean, this is even better than Perl for this kind of 
> text-processing
> code!
>
> The only complaint is that I couldn't write auto[string] dgs 
> and have
> the compiler auto-infer the delegate type. :-) Additionally, I 
> wasn't
> sure if I could omit the "delegate(string,string)" after each 
> keyword;
> if that's actually allowed, then this would make D totally pwn 
> Perl!!
>
> (I left out some stuff that makes this code even more of a joy 
> to write:
> using nested try/catch blocks, I can throw exceptions from 
> deep-down
> parsing code and have the loop that loops over input lines 
> automatically
> prefix error messages with the filename/line number where the 
> error
> occurred. This way, even errors thrown by to!int() will be 
> formatted
> nicely. With Perl, this gets extremely messy due to its 
> pathological use
> of $. for line numbers which can get overwritten in unexpected 
> places if
> you're processing more than one file at a time.)
>
> Did I mention I'm totally in love with D?? Seriously. It can 
> handle
> system-level code and "high-level" text-processing code with 
> total
> impunity. What's there not to like?!
>
>
> T