Does Scala have guards?
I started learning scala a few days ago and when learning it, I am comparing it with other functional programming languages like (H开发者_如何学Pythonaskell, Erlang) which I had some familiarity with. Does Scala has guard sequences available?
I went through pattern matching in Scala, but is there any concept equivalent to guards with otherwise
and all?
Yes, it uses the keyword if
. From the Case Classes section of A Tour of Scala, near the bottom:
def isIdentityFun(term: Term): Boolean = term match {
case Fun(x, Var(y)) if x == y => true
case _ => false
}
(This isn't mentioned on the Pattern Matching page, maybe because the Tour is such a quick overview.)
In Haskell, otherwise
is actually just a variable bound to True
. So it doesn't add any power to the concept of pattern matching. You can get it just by repeating your initial pattern without the guard:
// if this is your guarded match
case Fun(x, Var(y)) if x == y => true
// and this is your 'otherwise' match
case Fun(x, Var(y)) if true => false
// you could just write this:
case Fun(x, Var(y)) => false
Yes, there are pattern guards. They're used like this:
def boundedInt(min:Int, max: Int): Int => Int = {
case n if n>max => max
case n if n<min => min
case n => n
}
Note that instead of an otherwise
-clause, you simply specifiy the pattern without a guard.
The simple answer is no. It is not exactly what you are looking for (an exact match for Haskell syntax). You would use Scala's "match" statement with a guard, and supply a wild card, like:
num match {
case 0 => "Zero"
case n if n > -1 =>"Positive number"
case _ => "Negative number"
}
I stumbled to this post looking how to apply guards to matches with multiple arguments, it is not really intuitive, so I am adding an random example here.
def func(x: Int, y: Int): String = (x, y) match {
case (_, 0) | (0, _) => "Zero"
case (x, _) if x > -1 => "Positive number"
case (_, y) if y < 0 => "Negative number"
case (_, _) => "Could not classify"
}
println(func(10,-1))
println(func(-10,1))
println(func(-10,0))
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