#1482: unsafeCoerce# doesn't always fully coerce
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Reporter: [EMAIL PROTECTED] | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.6.1
Severity: normal | Resolution:
Keywords: unsafeCoerce# | Difficulty: Unknown
Os: Linux | Testcase:
Architecture: x86 |
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Comment (by Isaac Dupree):
LOL, I accidentally left out the ":t" in GHCi, and got (same result on x86
6.6.1)
{{{
> unsafeCoerce# shows
<interactive>: internal error: PAP object entered!
(GHC version 6.6.1 for powerpc_unknown_linux)
Please report this as a GHC bug:
http://www.haskell.org/ghc/reportabug
Aborted (core dumped)
}}}
What kind of showing is GHC trying to do on an object of type (forall b.
b) anyway?
Passing -fno-extended-default-rules didn't work for me to disable that in
ghci, so I went to a .hs file:
No, (shows::a -> String -> String) doesn't typecheck. (forall a. a ->
String -> String) isn't a type of `shows`, only (forall a. Show a => a ->
String -> String) is.
{{{
f :: forall a. a -> String -> String
f = undefined
e1 = ( unsafeCoerce# ) f
}}}
typechecks. Type variables that could be anything at all, don't need to
be instantiated.
{{{
e2 = (unsafeCoerce# :: (forall a. Show a => a -> String -> String) -> b)
shows
}}}
typechecks. GHC can't infer a rank-2 type for unsafeCoerce, but you can
give it one (`forall b. (forall a. Show a => a -> String -> String) -> b`
is clearly a more-specific version of unsafeCoerce's type `forall a b. a
-> b`). This allows the ''polymorphic'' shows to be passed to
unsafeCoerce, so that `shows` object can then be coerced as normal. (Rank-
two types with typeclasses aren't nice, you always have to specify a new
type signature when the set of type-classes is different...)
Neither -fno-monomorphism-restriction nor defaulting are needed.
Note that there is a difference between
{{{(unsafeCoerce# :: (Integer -> String -> String) -> b) shows}}}
(or whatever the type variable would default to, if not Integer)
and
{{{(unsafeCoerce# :: (forall a. Show a => a -> String -> String) -> b)
shows}}}
.
Their representations are not the same; the second one can only be coerced
back to the fully polymorphic version, and the first one can only be
coerced back to the Integer version (well, probably if you wanted to pass
_|_ for that parameter then you could also coerce the second one to have
that parameter's type be anything you wanted, including polymorphic, as
long as it wasn't qualified with a type-class constraint).
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1482>
GHC <http://www.haskell.org/ghc/>
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