Daily scala

Today's topic is related to Defining Custom Control Structures. By name parameters are not function objects in the same way as other functions. Normally you need to invoke a function:

1. scala> def exec(func: () => Int) = func()
2. exec: (() => Int)Int
3. // exec invokes the function so 10 is the result
4. scala> exec (() => 10)  
5. res1: Int = 10
6. scala> def exec(func: () => Int) = func
7. exec: (() => Int)() => Int
8. // here exec returns the function:
9. scala> exec (() => 10)                 
10. res2: () => Int = <function>

Compare that to a by-name-parameter function:

1. scala> def exec(func: => Int) = func   
2. exec: (=> Int)Int
3. // by-name-parameters are executed when they are referenced
4. // so the result is 10
5. scala> exec {10}                    
6. res3: Int = 10
7. // This is not legal because by-name-parameters
8. // are not normal functions
9. scala> def exec(func: => Int) = func()
10. <console>:4: error: func of type Int does not take parameters
11.        def exec(func: => Int) = func()

So the issue is how can you pass a by-name-parameter to a method that takes a function as a parameter without having to do:

1. scala> def takesFunc(func: () => Int) = println(func())
2. takesFunc: (() => Int)Unit
3. scala> def send(x: => Int) = takesFunc(() => x) 
4. send: (=> Int)Unit
5. scala> send{2}
6. 2

the alternative syntax is:

1. scala> def send(x: => Int) = takesFunc (x _)   
2. send: (=> Int)Unit
3. scala> send {2}
4. 2

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