Pattern matching with conjunctions (PatternA AND PatternB)
Scala has a language feature to support disjunctions in pattern matching ('Pattern Alternatives'):
x match {
case _: String | _: Int =>
case _ =>
}
However, I often need to trigger an action if the scrutiny satisfies PatternA and PatternB (conjunction.)
I created a pattern combinator '&&' that adds this capability. Three little lines that remind me why I love Scala!
// Splitter to apply two pattern matches on the same scrutiny.
object && {
def unapply[A](a: A) = Some((a, a))
}
// Extractor object matching first character.
object StartsWith {
def unapply(s: String) = s.headOption
}
// Extractor object matching last character.
object EndsWith {
def unapply(s: String) = s.reverse.headOption
}
// Extractor object matching length.
object Length {
def unapply(s: String) = Some(s.length)
}
"foo" match {
case StartsWith('f') && EndsWith('f') => "f.*f"
case StartsWith('f') && EndsWith(e) && Length(3) if "aeiou".contains(e) => "f..[aeiou]"
case _ => "_"
}
Points for discussion
- Is there an existing way to do this?
- Are there problems with this approach?
- Can this approach create any other useful combinators? (for example,
Not
) - Should such a combinator be added to the standard library?
UPDATE
I've just been asked how the compiler interprets case A && B && C
. These are infix operator patterns (Section 8.1.9 of the Scala Reference). You could also express this with standard extract patterns (8.1.7) as &&(&&(A, B), C).' No开发者_Go百科tice how the expressions are associated left to right, as per normal infix operator method calls like
Boolean#&&in
val b = true && false && true`.
I really like this trick. I do not know of any existing way to do this, and I don't foresee any problem with it -- which doesn't mean much, though. I can't think of any way to create a Not
.
As for adding it to the standard library... perhaps. But I think it's a bit hard. On the other hand, how about talking Scalaz people into including it? It looks much more like their own bailiwick.
A possible problem with this is the bloated translation that the pattern matcher generates.
Here is the translation of the sample program, generated with scalac -print
. Even -optimise
fails to simplify the if (true) "_" else throw new MatchError()
expressions.
Large pattern matches already generate more bytecode than is legal for a single method, and use of this combinator may amplify that problem.
If &&
was built into the language, perhaps the translation could be smarter. Alternatively, small improvements to -optimise
could help.
精彩评论