#7264: Adding GHC's inferred type signatures to a working program can make it
fail
with Rank2Types
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Reporter: guest | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.1
Resolution: fixed | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Difficulty: Unknown
Testcase: typecheck/should_fail/T7264 | Blockedby:
Blocking: | Related:
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Changes (by simonpj):
* status: new => closed
* testcase: => typecheck/should_fail/T7264
* resolution: => fixed
Comment:
Now GHC does not instantiate `mmap` at a polymorphic type, and we get this
error message from the program without the type signature
{{{
T7264.hs:11:19:
Couldn't match type `a' with `forall r. r -> String'
`a' is a rigid type variable bound by
the inferred type of mkFoo2 :: a -> Maybe Foo at T7264.hs:11:1
Expected type: a -> Foo
Actual type: (forall r. r -> String) -> Foo
Relevant bindings include
mkFoo2 :: a -> Maybe Foo (bound at T7264.hs:11:1)
val :: a (bound at T7264.hs:11:8)
In the first argument of `mmap', namely `Foo'
In the expression: mmap Foo (Just val)
In an equation for `mkFoo2': mkFoo2 val = mmap Foo (Just val)
}}}
It's not a fantastic message, but it's on the money, and (without
`-XImpredicativeTypes`) the program should certainly be rejected.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7264#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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