Is there a way in scala to get the arguments back from a already partially applied function?
Does this even make sense, should be done, or fits into any use case?
example:
def doStuff(lower:Int,upper:Int,b:String)=
for(turn <- lower to upper) println(turn +": "+b)
Imagine that at one point I know the 'lower' argument and I get a function of applying it to 'doStuff'
开发者_JS百科
val lowerDoStuff = doStuff(3,_:Int,_:String)
Is there a way for me to get that 3 back ? (for the sake of example, imagine that I am inside a function which only received 'lowerDoStuff' and now needs to know the first argument)
Idiomatic scala is prefered to introspection/reflection (if possible).
Idiomatic Scala: no, you can't. You have specifically said that the first argument is no longer relevant. If the compiler can make it disappear entirely, that's best: you say you have a function that depends on an int and a string, and you haven't made any promises about what generated it. If you really need that value, but you also really need to pass a 2-argument function, you can do it by hand:
class Function2From3[A,B,C,Z](f: (A,B,C) => Z, val _1: A) extends Function2[B,C,Z] {
def apply(b: B, c: C) = f(_1, b, c)
}
val lowerDoStuff = new Function2From3(doStuff _, 3)
Now when you get the function later on, you can pattern match to see if it's a Function2From3, and then read the value:
val f: Function2[Int,String,Unit] = lowerDoStuff
f match {
case g: Function2From3[_,_,_,_] => println("I know there's a "+g._1+" in there!")
case _ => println("It's all Greek to me.")
}
(if it's important to you that it be an integer, you can remove A
as a generic parameter and make _1
be an integer--and maybe just call it lower
while you're at it).
Reflection: no, you can't (not in general). The compiler's smarter than that. The generated bytecode (if we wrap your code in class FuncApp
) is:
public final void apply(int, java.lang.String);
Signature: (ILjava/lang/String;)V
Code:
0: aload_0
1: getfield #18; //Field $outer:LFuncApp;
4: iconst_3
5: iload_1
6: aload_2
7: invokevirtual #24; //Method FuncApp.doStuff:(IILjava/lang/String;)V
10: return
Notice the iconst_3
? That's where your 3 went--it disappeared into the bytecode. There's not even a hidden private field containing the value any more.
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