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Unsound subtyping behavior with (=> Any) #1386

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scabug opened this issue Sep 27, 2008 · 5 comments
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Unsound subtyping behavior with (=> Any) #1386

scabug opened this issue Sep 27, 2008 · 5 comments
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@odersky
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should not compile

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@scabug
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scabug commented Sep 27, 2008

The following is correctly disallowed:

scala> def foo(x: Int) = x + 1
foo: (Int)Int

scala> def bar(g: (=>Int) => Int, x: Int) = g(x)
bar: ((=> Int) => Int,Int)Int

scala> bar(foo, 3)
<console>:7: error: type mismatch;
 found   : (Int) => Int
 required: (=> Int) => Int
       bar(foo, 3)
           ^

but at present this seems to work:

scala> def myToString(x : Any) : String = x.toString   
foo: (Any)String

scala> def bar(g:(=>Any) => String, x:Int) = g(x)
bar: ((=> Any) => String,Int)String

scala> bar(myToString, 3)
res13: String = <function>
@scabug
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scabug commented Sep 27, 2008

Imported From: https://issues.scala-lang.org/browse/SI-1386?orig=1
Reporter: Geoffrey Alan Washburn (washburn)
See #5517

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scabug commented Sep 27, 2008

@odersky said:
I fail to see the unsoundness. (=> Any) is a subtype of Any.

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scabug commented Sep 27, 2008

Geoffrey Alan Washburn (washburn) said:
If (=> Any) is a subtype of Any, then why is (=> Int) not a subtype of Int?

Maybe you didn't look closely enough, but clearly the abstractions are getting violated because in the latter example myToString is receiving some kind of thunk rather than the integer 3 as its argument.

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scabug commented Sep 27, 2008

Aaron Harnly (aaronharnly) said:
Replying to [comment:2 washburn]:

If (=> Any) is a subtype of Any, then why is (=> Int) not a subtype of Int?

Eh? (=> Any) is a subtype of Any in the same sense that (=> Int) is a subtype of Any.

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scabug commented Sep 27, 2008

Geoffrey Alan Washburn (washburn) said:
Replying to [comment:3 aaronharnly]:

Replying to [comment:2 washburn]:

If (=> Any) is a subtype of Any, then why is (=> Int) not a subtype of Int?

Eh? (=> Any) is a subtype of Any in the same sense that (=> Int) is a subtype of Any.

Indeed, I hadn't been thinking about the problem in that fashion.

However, there is still the issue that the abstraction boundaries are getting violated. At least with respect the current description of by-name types/parameters in the specification.

@scabug scabug closed this as completed Dec 13, 2012
This was referenced Apr 7, 2017
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@scala scala deleted a comment from scabug Jan 21, 2020
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