def compose [A，B，C]（f：B => C，g：A => B）：A => C = {f（g（_））}是不是有效的Scala声明？

Question

I am new to scala this might be a cake walk concept but trying to understand why

def compose[A,B,C](f: B => C, g: A => B): A => C = {f(g(_))}

Is not valid scala declation?

Answer 1

Try

def compose[A,B,C](f: B => C, g: A => B): A => C = { a: A => f(g(a)) }

The underscore-as-placeholder syntax works inconsistently, and in particular doesn't tend to work in nested function calls like you have here. A good rule of thumb is that if the underscore syntax is erroring, try expanding it to an explicit lambda and see if it works then.

Answer 2

因为f(g(_))在Scala中表示f(x => g(x)) ，而不是x => f(g(x)) 。

Answer 3

You need to assign a name to the variable that receives your function g:

def compose[A,B,C](f: B => C, g: A => B): A => C = {x => f(g(x))}

You can also do as follows (i renamed the function name to make it clear that is a differente thing) :

def myCompose[A,B,C](f: B => C, g: A => B): A => C = f compose g