#5529: Newtypes with hidden constructors cannot be passed as FFI arguments
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Reporter: mikhail.vorozhtsov | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.3
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by mikhail.vorozhtsov):
Replying to [comment:10 simonmar]:
> I think your point (1) is saying that even if we know that `CTime ==
Int64`, what the programmer actually needs to know is that `CTime ==
time_t`, correct?
Yes. And, more importantly, the programmer needs a way to ensure that
`CTime == time_t` ''automatically'' at compile time, because we can't
really expect people to recheck that equality by hand every time a new
version of the library is released/used. This hypothetical machinery must
be able to handle multiple levels of wrapping, like `CTime -> X1 -> X2 ->
Int64`. My other (original) concern is that exporting a constructor does
not only reveal the representation of the type, but also, obviously, gives
third party non-FFI code a way to construct instances of the type. For
CTime it may be ok, but think about flag types, which are allowed to have
only limited number of bits set (or, even worse, only limited number of
combinations of set bits).
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5529#comment:11>
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