#816: Weird fundep behavior (with -fallow-undecidable-instances)
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Reporter: nibro | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler (Type checker) | Version: 6.4.2
Severity: normal | Keywords:
Os: Unknown | Difficulty: Unknown
Architecture: Unknown |
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I encounter a strange behavior with functional dependencies. Assume we
have a class defined as
class Foo x y | x -> y where
foo :: x -> y
and another class
class Bar x y where
bar :: x -> y -> Int
and I want to write the instance declaration
instance (Foo x y, Bar y z) => Bar x z where
bar x z = bar (foo x) z
Compiling (with 6.4.2, -fallow-undecidable-instances and -fglasgow-exts) I
get the following error message:
<error>
Foo.hs:12:0:
Context reduction stack overflow; size = 21
Use -fcontext-stack20 to increase stack size to (e.g.) 20
`$dBar :: Bar y z' arising from use of `bar' at Foo.hs:13:11-13
[... same thing 20 times ...]
`$dBar :: Bar y z' arising from use of `bar' at Foo.hs:13:11-13
`bar :: {bar at [y z]}' arising from use of `bar' at Foo.hs:13:11-
13
When trying to generalise the type inferred for `bar'
Signature type: forall x y z. (Foo x y, Bar y z) => x -> z ->
Int
Type to generalise: x -> z -> Int
In the instance declaration for `Bar x z'
</error>
The declaration requires undecidable instances, but I doubt that the
problem comes from that. What makes it even more weird is that I can get
this to compile, and behave as expected, if I do one of a) declare an
instance of Bar for any type, or
b) add an explicit type signature (foo x :: y) in the definition of Bar.
The first seems weird since how could a different, unrelated instance
affect the typeability of the second instance? The second, b), is weird
since by the FD x -> y we should already know that foo x :: y.
Same behavior in GHC 6.4.1. Hugs (with -98 +O) accepts the code.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/816>
GHC <http://www.haskell.org/ghc/>
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