#2935: "A lazy (~) pattern cannot bind existential type variables" happens for
non-existential GADTs
---------------------------------+------------------------------------------
Reporter: ganesh | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 6.10.1
Severity: normal | Resolution: wontfix
Keywords: | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
---------------------------------+------------------------------------------
Changes (by simonpj):
* status: new => closed
* difficulty: => Unknown
* resolution: => wontfix
Comment:
The error message is not good, but it's right to reject this program.
More precisely, it'd definitely be right to reject this program:
{{{
bar :: a -> Foo a -> a
bar x ~(Foo _) = fst x
}}}
Because I could call `bar 3 undefined`, and thereby crash. Now in this
case (a) the data type has only one constructor, (b) the type signature
for `foo` ensures that every call will match `Foo`'s constraints. So
nothing would go wrong. But it's a very special case. And I don't know
how to translate it into System F (or more precisely Fc).
So it's not easy. So I propose to do nothing except perhaps make the
error message mention GADTs too.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2935#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
