#3819: keeping RelaxedPolyRec as optional feature can help spotting infinite
recursion
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Reporter: guest | Owner:
Type: proposal | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 6.12.1 | Keywords:
Os: Unknown/Multiple | Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
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This is a ticket you have nothing to do about - isn't this great?
I just want to give a real-world case where the current behaviour helped
me to detect an infinite recursion early.
I wrote the following code, that implements a {{{poke}}} for any
{{{Traversable}}} container with {{{Storable}}} elements:
{{{
poke ::
(Fold.Foldable f, Storable a) =>
Ptr (f a) -> f a -> IO ()
poke ptr x =
evalStateT (Fold.traverse_ pokeState x) $
castPtr ptr
pokeState ::
(Storable a) =>
a -> StateT (Ptr a) IO ()
pokeState x = do
liftIO . flip poke x =<< get
modify (flip advancePtr 1)
}}}
You can find this code here:
http://code.haskell.org/~thielema/storable-
record/src/Foreign/Storable/Traversable.hs
When I compiled this I got the compiler error:
{{{
src/Foreign/Storable/Traversable.hs:67:0:
Contexts differ in length
(Use -XRelaxedPolyRec to allow this)
When matching the contexts of the signatures for
poke :: forall (f :: * -> *) a.
(Fold.Foldable f, Storable a) =>
Ptr (f a) -> f a -> IO ()
pokeState :: forall a. (Storable a) => a -> StateT (Ptr a) IO ()
The signature contexts in a mutually recursive group should all be
identical
When generalising the type(s) for poke, pokeState
}}}
This quickly pointed me to the problem, that the call to {{{poke}}} in
{{{pokeState}}} actually was wrong. It must be {{{Storable.poke}}}. If GHC
had compiled this, it would have certainly gone into an infinite
recursion.
There are two ways to treat this example:
* Blame my naming style where I re-use common identifiers and distinguish
them later by qualification.
* Count it as vote for keeping the RelaxedPolyRec as optional feature.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3819>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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