Re: Partially applied function error - OK in foldLeft

While that is no real reason, just the difference: The use in foldLeft
expects a return type of Map[Int,String => String], while the  second
use doesn't expect any (specific) return type. I would guess the first
use helps the type inferer to guess what you meant to say. Still, the
syntax Scala requires in the second case is the general one: 'seed(1)
= z _', i.e. treat the method z as (unapplied) function object.

On Tue, Jul 7, 2009 at 12:39 PM, Jem<jem.mawson@...> wrote:
> So how is acc(next) = z  different to seed(1) = z ?
> The only difference
> is that one is within the block parameter of List.foldLeft and the other is
> stand-alone.
> In my observation what triggers treatment as call-by-name is the typing of
> the method or container that is receiving the reference.
> The snippet seed(1) = z is assigning z: String=>String as a value in a map
> that takes values of type String=>String. There's no ambiguity as far as I
> can tell.
>
>
>
> 2009/7/7 Jem <jem.mawson@...>
>>
>> Thanks Mario,
>> So how is acc(next) = z  different to seed(1) = z ?
>> The only difference
>> is that one is within the block parameter of List.foldLeft and the other is
>> stand-alone.
>>
>>
>>
>> 2009/7/7 Mario Camou <mcamou@...>
>>>
>>> Hi Jem,
>>>
>>> seed(1)=z means "call function z and assign the return value". The
>>> principle of uniform access means that Scala doesn't require parentheses on
>>> method calls with no arguments. To get around this you use the underscore to
>>> indicate you want to rederence the function object itself.
>>>
>>> Hope this helps,
>>> -Mario.
>>>
>>> - Sent from my Android G1
>>>
>>> On Jul 7, 2009 9:31 AM, "Jem" <jem.mawson@...> wrote:
>>>
>>> Hi. I'm not clear on why these two scenarios are different. Can someone
>>> explain why the foldLeft code works, but the map(i)=z operation fails?
>>>
>>> scala> def z(s: String) = s
>>> z: (String)String
>>> scala> val seed = Map.empty[Int, (String)=>String]
>>> seed: scala.collection.immutable.Map[Int,(String) => String] = Map()
>>> scala> List(1,2,3).foldLeft(seed) {(acc, next) => acc(next) = z}
>>>
>>> res23: scala.collection.immutable.Map[Int,(String) => String] = Map(1 ->
>>> <function>, 2 -> <function>, 3 -> <function>)
>>>
>>> scala> seed(1) = z
>>> <console>:7: error: missing arguments for method z in object $iw;
>>> follow this method with `_' if you want to treat it as a partially
>>> applied function
>>>        seed(1) = z
>>>                  ^
>>>
>>> I know z _ will work, as per the message, but I don't understand how the
>>> latter operation differs from the foldLeft scenario. In both cases I invoke
>>> map(int)=<function>. What is the difference?
>>> Thanks
>>> Jeremy
>
>

--
Johannes

-----------------------------------------------
Johannes Rudolph
http://virtual-void.net