#5015: Can't unsafeCoerce a GADT with a coercion
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Reporter: TristanAllwood | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.0.2 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by diatchki):
This is not a bug, GHC is working as expected.
The constructor {{{E}}} has a polymorphic type, which is automatically
instantiated when you use it. This means that every use of {{{E}}} will
be at a monomorphic type of the form {{{a0 -> E}}} for some type {{{a0}}}
and, one the side, GHC has to check that {{{Blah a0 ~ Int}}}.
In these examples there is no reason to believe that {{{Blah a0}}} is
{{{Int}}}, which is what the type error says. You could fix this by
specifying the instantiation type explicitly. For example,
{{{unsafeCoerce (E :: Int -> E)}}} will probably type-check (although it
will probably crash if you tried to use it).
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5015#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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