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Type system explanation needed
Thu, 2011-10-27, 00:56
Hello,
I have an interesting (I guess) case, that I expected the compiler to
handle, but it cannot do it. I'm wondering if it is possible to handle
it somehow:
scala> class Holder[T](var f1 : T, var f2 : T)
scala> val str = new Holder("t1", "t2")
str: Holder[java.lang.String] = Holder@5b202f4d
scala> val num = new Holder(5, 10)
num: Holder[Int] = Holder@7f8b70fb
scala> List(str, num).foreach(h => h.f1 = h.f2)
:11: error: type mismatch;
found : Any
required: Int with java.lang.String
List(str, num).foreach(h => h.f1 = h.f2)
^
To summarise I want to iterate over all Holders whatever T they have
and reassign one field from the other. Although I don't know their
exact types I know for sure, that they have the same type. I know,
that type erasure works, so it fails, although I would argue, that the
compiler might be smart enough here.
So I tried this way, and it is even more surprising to me:
scala> class Holder[_T](_f1 : _T, _f2 : _T) {
| type T = _T
| var f1 : T = _f1
| var f2 : T = _f2
| }
defined class Holder
scala> val str = new Holder("t1", "t2")
str: Holder[java.lang.String] = Holder@1b92d8d6
scala> val num = new Holder(5, 10)
num: Holder[Int] = Holder@460c5dbb
scala> List(str, num).foreach(h => h.f1 = h.f2)
scala> List(str, num).foreach(h => h.f1 = new Thread())
scala> str.f1
java.lang.ClassCastException: java.lang.Thread cannot be cast to
java.lang.String
So now, although scala compiler knows nothing more than in the
previous case, it accepts the h.f1 = h.f2 assignment. Moreover it even
accepts totally wrong assignments to Thread objects! And all of this
without any warning.
So my 2 questions are:
1) How to be sure, that in such case I can assign h.f1 = h.f2 as types
are the same whatever they are, but at the same time I cannot assign
whatever.
2) Is second behaviour proper? If so, then why?
Thanks,
Roman
Thu, 2011-10-27, 04:07
#2
Re: Type system explanation needed
On Wed, Oct 26, 2011 at 4:56 PM, RomKal wrote:
> 2) Is second behaviour proper? If so, then why?
I believe you have found larvae in the ointment.
Thu, 2011-10-27, 10:47
#3
Re: Type system explanation needed
On Thu, Oct 27, 2011 at 12:56 AM, RomKal wrote:
> So I tried this way, and it is even more surprising to me:
>
> scala> class Holder[_T](_f1 : _T, _f2 : _T) {
> | type T = _T
> | var f1 : T = _f1
> | var f2 : T = _f2
> | }
> defined class Holder
>
> scala> val str = new Holder("t1", "t2")
> str: Holder[java.lang.String] = Holder@1b92d8d6
>
> scala> val num = new Holder(5, 10)
> num: Holder[Int] = Holder@460c5dbb
>
> scala> List(str, num).foreach(h => h.f1 = h.f2)
>
> scala> List(str, num).foreach(h => h.f1 = new Thread())
>
> scala> str.f1
> java.lang.ClassCastException: java.lang.Thread cannot be cast to
> java.lang.String
Ouch! Very, nasty :-(
Cheers,
Miles
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Am 27.10.2011 01:56, schrieb RomKal:
Your holder, but a little prepared:
class Holder [T] (var f1 : T, var f2 : T) {
def reassign () {
f1 = f2
}
}
val str = new Holder ("t1", "t2")
val num = new Holder (5, 10)
// fails:
List (str, num).foreach (h => h.f1 = h.f2)
// prepared method:
List (str, num).foreach (_.reassign)
// a separate method ...
def holderReassign [T] (h: Holder[T]) = {
h.f1 = h.f2 }
// holderReassign: [T](h: Holder[T])Unit
//... works too:
List (str, num).foreach (h => holderReassign (h))
> So my 2 questions are:
> 1) How to be sure, that in such case I can assign h.f1 = h.f2 as types
> are the same whatever they are, but at the same time I cannot assign
> whatever.
I hope it takes you a step forward.
> 2) Is second behaviour proper? If so, then why?
Hm. I guess it is type inference. From a List (str, num), the REPL
concludes that you have a List of Holder[Any], because Any is the first
common subtype, and a Holder of Any might exist.
If you declare str explicitly as Holder of Any:
val str = new Holder[Any] ("t1", "t2")
str: Holder[Any] = Holder@3d16b2
scala> str.f1 = new Thread ()
// works fine, no Problem.