Why is java.util.UUID.randomUUID not accepted as a parameter

[Cecil Westerhof]

I have the following function definition:

def testGenerateUuids(name: String,
uuidFn: () => String,
repeats: Int,
iterations: Int) {

When I call this with:
testGenerateUuids("uuid", java.util.UUID.randomUUID, 5, 1000000)
I get:
/home/cecil/scala/test.scala:111: error: type mismatch;
found : java.util.UUID
required: String
testGenerateUuids("uuid", java.util.UUID.randomUUID, 5, 1000000)
^
What am I doing wrong?

--
Cecil Westerhof
Senior Software Engineer
LinkedIn: http://www.linkedin.com/in/cecilwesterhof

[√iktor Ҡlang]

On Sun, Sep 9, 2012 at 12:26 PM, Cecil Westerhof <cldwes...@gmail.com> wrote:

I have the following function definition:

    def testGenerateUuids(name:       String,
                          uuidFn:     () => String,
                          repeats:    Int,
                          iterations: Int) {

When I call this with:
    testGenerateUuids("uuid", java.util.UUID.randomUUID, 5, 1000000)
I get:
    /home/cecil/scala/test.scala:111: error: type mismatch;
     found   : java.util.UUID
     required: String
    testGenerateUuids("uuid", java.util.UUID.randomUUID, 5, 1000000)
                                             ^
What am I doing wrong?

What the compiler tells you: you are giving it a java.util.UUID and it wants a String.

Cheers,

√

 

--
Cecil Westerhof
Senior Software Engineer
LinkedIn: http://www.linkedin.com/in/cecilwesterhof

--
Viktor Klang

Akka Tech Lead

Typesafe - The software stack for applications that scale

Twitter: @viktorklang

[Lukas Rytz]

functions with an empty param list can be called without argument list "()".

so "java.util.UUID.randomUUID" is an application, you need

"() => java.util.UUID.randomUUID"

"java.util.UUID.randomUUID _"

On Sun, Sep 9, 2012 at 12:26 PM, Cecil Westerhof <cldwes...@gmail.com> wrote:

[Lukas Rytz]

oh, victor is right of course, you need .toString, and ignore my answer, the

compiler will do the eta expansion for you based on the parameter type.

[Cecil Westerhof]

Op zondag 9 sep 2012 12:34 CEST schreef √iktor Ҡlang:

> On Sun, Sep 9, 2012 at 12:26 PM, Cecil Westerhof <cldwes...@gmail.com> wrote:
>
> I have the following function definition:
>
>     def testGenerateUuids(name:       String,
>                           uuidFn:     () => String,
>                           repeats:    Int,
>                           iterations: Int) {
>
> When I call this with:
>     testGenerateUuids("uuid", java.util.UUID.randomUUID, 5, 1000000)
> I get:
>     /home/cecil/scala/test.scala:111: error: type mismatch;
>      found   : java.util.UUID
>      required: String
>     testGenerateUuids("uuid", java.util.UUID.randomUUID, 5, 1000000)
>                                              ^
> What am I doing wrong?
>
> What the compiler tells you: you are giving it a java.util.UUID and it wants a
> String.

But why does it wants a String? I would think that with:
uuidFn: () => String,
it wants a function without parameters that returns a String.

[Cecil Westerhof]

Op zondag 9 sep 2012 12:42 CEST schreef Cecil Westerhof:

Even more weird. The following works:

def nano() = {
println("Getting nano")
System.nanoTime
}

def delayed(t: => Long) = {
println("In delayed method")
println("Param: "+t)
t
}

delayed(nano)

But this does not:

def uuid() = {
println("Getting uuid")
java.util.UUID.randomUUID
}

def delayed(t: => String) = {
println("In delayed method")
println("Param: "+t)
t
}

delayed(uuid)

For my inexperienced eye it is the same. So why does the first work,
and the second not?

[AGYNAMIX Torsten Uhlmann]

You uuid() method returns a java.util.UUID, not a String.

You can make it return a String though: java.util.UUID.randomUUID.toString

scala> def uuid2() = {

     | java.util.UUID.randomUUID.toString

     | }

uuid2: ()java.lang.String

scala> delayed(uuid2)

In delayed method

Param: fc535715-3f72-469e-83a1-6827e4ed5ec0

res3: String = 198668e5-d367-4782-9af2-208ae87bcd41

Does that help?

Torsten.

-- 

AGYNAMIX(R). Passionate Software.
Inh. Torsten Uhlmann | Buchenweg 5 | 09380 Thalheim
Phone:       +49 3721 273445
Fax:             +49 3721 273446
Mobile:       +49 151 12412427
Web:           http://www.agynamix.de

[Cecil Westerhof]

Op zondag 9 sep 2012 13:50 CEST schreef AGYNAMIX Torsten Uhlmann:

> You uuid() method returns a java.util.UUID, not a String.
>
> You can make it return a String though: java.util.UUID.randomUUID.toString

Stupid mistake. I now changed it to:

def testGenerateUuids(name: String,
uuidFn: => java.util.UUID,
repeats: Int,
iterations: Int) {

By the way
uuidFn: () => java.util.UUID,
what I had seen somewhere does not work.


> Does that help?

It certainly did. Thanks.

[AGYNAMIX Torsten Uhlmann]

Glad I could help.

Now when you use param: => String its a call-by-name parameter which is evaluated each time it is accessed, 

if you use ()=>String then it's a function that gets no input parameter and returns a String.

To do the code below with a function, you could write:

def uuid() = {
       println("Getting uuid")
       java.util.UUID.randomUUID

}

def delayed(func: ()=>java.util.UUID) = {

       println("In delayed method")

       func()
}

delayed(()=>uuid()), or 

delayed(uuid _)

In the first "delayed" call you create a function that calls, when executed, the uuid() method (it's executed with the func() call in def delayed).

The second version lifts the "uuid" method into a function. In the case above you could omit the underscore, I'm not quiet sure about the rules when it's needed and when not.

Does that help even more? :)

Torsten.

-- 

AGYNAMIX(R). Passionate Software.
Inh. Torsten Uhlmann | Buchenweg 5 | 09380 Thalheim
Phone:       +49 3721 273445
Fax:             +49 3721 273446
Mobile:       +49 151 12412427
Web:           http://www.agynamix.de

[Cecil Westerhof]

Op zondag 9 sep 2012 15:11 CEST schreef AGYNAMIX Torsten Uhlmann:

> Glad I could help.
>
> Now when you use param: => String its a call-by-name parameter which is
> evaluated each time it is accessed,
>
> if you use ()=>String then it's a function that gets no input parameter and
> returns a String.

I used the previous version, but the function was just called as I
wanted. Why (if at all) should I switch to this notation?

> Does that help even more? :)

My head starts to hurt. :-D

[AGYNAMIX Torsten Uhlmann]

Am 09.09.2012 um 15:49 schrieb Cecil Westerhof <cldwes...@gmail.com>:

> Op zondag 9 sep 2012 15:11 CEST schreef AGYNAMIX Torsten Uhlmann:
>
>> Glad I could help.
>>
>> Now when you use param: => String its a call-by-name parameter which is
>> evaluated each time it is accessed,
>>
>> if you use ()=>String then it's a function that gets no input parameter and
>> returns a String.
>
> I used the previous version, but the function was just called as I
> wanted. Why (if at all) should I switch to this notation?

The different notations are for different purposes. A call-by-name parameter is a parameter to a function that's evaluated on every access.
If you write a log method for instance, these parameters come in handy:

def log(msg: => String): Unit = {
if (isLogEnabled) println(msg)
}

If you call it like log("%s has the value of %s".format(valueA, valueB)) then the string is only assembled if "isLogEnabled" is true.

The other notation ()=>String is for passing blocks of code (functions) into objects, classes, methods, etc. You can pass parameters:

String=>String for instance which you can not do with a call-by-name parameter.

Now, I would guess, that a call-by-name parameter and a function with no parameters will both be compiled into a Function0 (that's a Trait that has an apply method that returns what your function would return or what your parameter type is), but I don't know if there are internal differences.

So using call-by-name conveys a different message than using a function block.

Here's an example with a function that takes a parameter:

scala> def uuid(prefix: String)={
| println("Getting uuid")
| prefix+java.util.UUID.randomUUID
| }
uuid: (prefix: String)java.lang.String

scala> def delayed(prefix: String, func: String=>String)={

| println("In delayed method")

| func(prefix)
| }
delayed: (prefix: String, func: String => String)String

scala> delayed("Hi_", uuid)
In delayed method
Getting uuid
res1: String = Hi_d40649f1-c417-43dd-87e4-35bd2121fc47

Hope I could clear up the head spinning :)

Torsten.