Re: Re: Strange type error

> I can't say that I understand the why any better

http://www.scala-lang.org/files/archive/spec/2.11/06-expressions.html#placeholder-syntax-for-anonymous-functions

which includes brief examples. There are also answers on stackoverflow. You
might contrast:

scala> val f: (Int, Int) => Any = (_, _)
f: (Int, Int) => Any = <function2>

scala> val f: (Int, Int) => Any = (_, _ + 1)
<console>:7: error: missing parameter type for expanded function ((x$1) =>
scala.Tuple2(x$1, ((x$2) => x$2.$plus(1))))

You can cheat:

scala> val f: (Int,Int,Int) => Int = _ + _ $times _
f: (Int, Int, Int) => Int = <function3>

scala> f(1,2,3)
res5: Int = 9



On Thu, May 7, 2015 at 8:15 AM, Toby <[email protected]> wrote:

> Ok, so in essence when building the tree the compiler does not yet realise
> that it requires a three-parameter function at that point and hence follows
> the 'default approach' for combining low-level stuff into higher level
> stuff. The brackets change what that default is.
>
> I can't say that I understand the why any better, but I suspect that that
> is not a 'user-level' issue. Sometimes, there is just too much sugar!
>
> On Thursday, 7 May 2015 16:50:35 UTC+2, Oliver Ruebenacker wrote:
>>
>>
>>      Hello,
>>
>>   First, the compiler needs to turn the source code into a syntax tree.
>> Only after that, it assigns types to nodes of the tree.
>>
>>   So, at the time the compiler needs to decide which part of the code
>> constitutes an anonymous function, it cannot yet take into account, what
>> type is required. It needs to go by syntax only, and that leaves few
>> options besides limiting it to simple expressions.
>>
>>   Something like (_ + _) * 3 is not necessarily illegal, there may be an
>> implicit conversion of a function into something that has a * operator.
>>
>>      Best, Oliver
>>
>> On Thu, May 7, 2015 at 10:31 AM, Toby <[email protected]> wrote:
>>
>>> Thank you Janusz and Phillips for your answers.
>>>
>>> However, you have told me what happens but not why it happens. I am
>>> hence a little wiser but no more enlightened.
>>>
>>> Why does the compiler decide that the presence of brackets requires
>>> nested functions, rather than one function with multiple parameters and
>>> precedence? The unique type that would satisfy the requirements of the
>>> function being called is a function of three Double parameters. This makes
>>> no sense to me.
>>>
>>> On Thursday, 7 May 2015 13:10:28 UTC+2, Andrew Phillips wrote:
>>>>
>>>> > but with additional brackets to change precedence, (_ + _) * _ fails
>>>>
>>>> Just for completeness: as you probably also discovered, it also fails
>>>> if you *don't* affect the precedence, and simply add brackets for e.g.
>>>> clarity:
>>>>
>>>> scala> execute( _ + (_ * _) )
>>>> <console>:9: error: missing parameter type for expanded function ((x$1)
>>>> => x$1.$
>>>> plus(((x$2, x$3) => x$2.$times(x$3))))
>>>>               execute( _ + (_ * _) )
>>>>                        ^
>>>>
>>>> Regards
>>>>
>>>> ap
>>>>
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>>
>>
>>
>> --
>> Oliver Ruebenacker
>> Solutions Architect at Altisource Labs <http://www.altisourcelabs.com/>
>> Be always grateful, but never satisfied.
>>
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