`Unit`返回关闭类型

Question

Given the following closure:

scala> def foo(x: Int) { 
     |   def bar(y: Int) = x + y
     |   bar(55)
     | }
foo: (x: Int)Unit

Why is its return type Unit ? I would've expected an Int .

scala> foo(55)
scala> 

Answer 1

You are using the procedure syntax. ie you don't have an = after the method declaration, so the method will return Unit . What you want is:

scala> def foo(x: Int) = { 
     |     def bar(y: Int) = x + y
     |     bar(55)
     | }
foo: (x: Int)Int

The value of bar(55) in your example is being discarded, which you can catch via compiler flag:

$ scala -Ywarn-value-discard

scala> def foo(x: Int) { 
     |     def bar(y: Int) = x + y
     |     bar(55)
     | }
<console>:9: warning: discarded non-Unit value
           bar(55)
              ^
foo: (x: Int)Unit