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Re: Partially applied functions
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Posted: May 29, 2016 10:40 PM
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What the author meant was you can't assign a function to a var or val. See line 10 below.
It seems you can pass a function to another function. However, that's only a "special case". A few pages later, the author said "If you are writing a partially applied function expression in which you leave off all parameters, such as println _ or sum _, you can express it more concisely by leaving off the underscore if a function is required at that point in the code". In other words, when you pass a function as a partially applied function to another function, you can pass it as is. (again, this is only a compiler trick, you are actually still passing square _, see line 17 below)
To be honest, I think this is kind of lame. In almost all functional programming languages, you can assign functions to vals or vars without the ugly trailing underscore.
1 object PartiallyAppliedFunction {
2 def square(x:Int) = x * x
3
4 def sumOfSquares(x:Int=>Int, a:Int, b:Int) = x(a) + x(b)
5
6 def main(args:Array[String]) {
7 println("hello world");
8
9 // this line won't compile
10 // val squareAlias = square
11
12 // this line will compile
13 val squareAlias = square _
14
15 println("square of 3 is: " + squareAlias(3))
16
17 println(sumOfSquares(square _, 1,2))
18 }
19
20 }
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