如何使用显式返回类型定义Scala闭包

Question

Why doesn't this work in scala:

val cloz: (Int,String => String) = (num: Int, str: String) => {
    str+"-"+num
}

I see a few examples of closures being defined with only 1 arg, like this:

val thingy: (Int => Int) = (num: Int) => {
    num * 2
}

But absolutely nowhere (including Scala ebooks) could I find any information explaining the syntax of "val" closures.

Thanks! Jamie

Answer 1

The correct syntax is:

val cloz: (Int, String) => String = (num: Int, str: String) => {
    str + "-" + num
}

By the way, in this simple case you can also simplify expression like this (especially if you already explicitly specifying the type of the function):

val cloz: (Int, String) => String = (num, str) => str + "-" + num 

---

Update

You can also use REPL to explore Scala - it's very nice tool. You can start it just by starting scala without any arguments. Here is example session:

scala> val cloz = (num: Int, str: String) => str + "-" + num
cloz: (Int, String) => java.lang.String = <function2>

scala> val cloz: (Int, String) => String = (num: Int, str: String) => {
     |     str + "-" + num
     | }
cloz: (Int, String) => String = <function2>

scala> val cloz: (Int, String) => String = (num, str) => str + "-" + num
cloz: (Int, String) => String = <function2>

scala> def printCloz(cloz: (Int, String) => String, num: Int, str: String) = print(cloz(num, str))
printCloz: (cloz: (Int, String) => String, num: Int, str: String)Unit

As you can see it not only allows you to interactively execute code, but also prints type information if you define something.

Answer 2

Based on the excellent answer from @Eazy Angel, I offer this for anyone who's confused as I was:

val cloz: (Int,String) => String = (num, str) => {
    str+"-"+num
}
def f1(  clozArg: ((Int,String) => String), intArg: Int, stringArg: String  ): String = {
    clozArg(intArg,stringArg)
}
println("f1 result="+f1(cloz, 5, "okee"))

Please note however (please correct me if I'm wrong) that b/c of Scala's type inference you are not required to specify all these types explicitly. The only time I have seen so far that you must do is when using recursion.

--
UPDATE:
This bizarre syntax works also: (see return type on 3rd line)

val cloz = (num: Int, str: String) => {
    str+"-"+num
} : String
def f1(  clozArg: ((Int,String) => String), intArg: Int, stringArg: String  ): String = {
    clozArg(intArg,stringArg)
}
println("f1 result="+f1(cloz, 5, "okee"))