Created
June 23, 2010 11:11
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| // scala @ syntax in patternmatching implemented as a library | |
| object `@` { | |
| def unapply[A](a:A) = Some(a, a) | |
| } | |
| // usage | |
| val Nums = """(\d),(\d)""".r | |
| "1,2" match { | |
| case a `@` Nums(first, second `@` "2") => println(a + "-"+first + "-" + second) | |
| } | |
| // prints | |
| 1,2-1-2 | |
| // it's actually more general | |
| "1,2" match { | |
| case Nums(a, b) `@` Nums(c, d) => println(List(a,b,c,d)) | |
| } | |
| //prints | |
| List(1, 2, 1, 2) | |
| // ----- | |
| "1,2" match { | |
| case Nums(a, b) @ Nums(c, d) => println(List(a,b,c,d)) | |
| } | |
| error: '=>' expected but '@' found. |
@retronym thanks for the links.. Love that hand rolled matcher!
Does anyone know why @ is part of the patternmatching syntax, when it can be implemented as a library so easily ?
Actually case x => is syntactic sugar for case x @ _ =>. I showed Martin Odersky my &&[A, B] a couple weeks ago and he was a little surprised to see that this could be implemented as a library! So Scala can really suprise all of us, usually in good ways :)
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See also:
"Pattern Matching with Conjunctions": http://stackoverflow.com/questions/2261358/pattern-matching-with-conjunctions-patterna-and-patternb
"Hand rolled pattern matcher": http://gist.github.com/376773