对scala函数文字感到困惑

Question

I confuse why the stringop3 is error. If I want to define value stringop3 has two params, one is a:String, the other is f:String=>String , what should I do.

// right
def stringop (a:String)(f:String=>String) = f(a)
// right
val stringop2=((a:String),(f:String=>String))=>f(a)
// error
val stringop3=(a:String)(f:String=>String)=>f(a)

the error is : error: not a legal formal paramete r.

Note: Tuples cannot be directly destructured in method or function parameters.

Either create a single parameter accepting the Tuple1,

or consider a pattern matching anonymous function: `{ case (param1, param1 ) => ... }

val stringop3=(a:String)(f:String=>String)=>f(a)

                      ^

one error found

Answer 1

Curried function definitions (eg: fn(a: A)(b: B): C ) can be visualised as fn: A => B => C.

So, you can define your stringops3 like this:

scala> val stringops3 = (a:String) => (f: String => String) => f(a)
stringops3: String => ((String => String) => String) = <function1>

scala> stringops3("foo")(_.toUpperCase)
res1: String = FOO