#1133: auto-derivation of Enum is disallowed with recursive imports
----------------------+-----------------------------------------------------
Reporter: heatsink | Owner: simonpj
Type: bug | Status: new
Priority: lowest | Milestone: _|_
Component: Compiler | Version: 6.6
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Unknown
Os: Unknown |
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Changes (by simonpj):
* milestone: 6.6.1 => _|_
* priority: normal => lowest
Comment:
The error message is certainly misleading. It now says:
{{{
Can't make a derived instance of `Enum X'
(even with cunning newtype deriving:
the newtype may be recursive)
In the newtype declaration for `X'
}}}
The problem is this. Generally, the "newtype deriving" mechanism is
cautious about recursive newtypes. Consider
{{{
newtype A = MkA [A]
}}}
We do not want to generate the instance
{{{
instance Eq [A] => Eq A
}}}
In this case the newtype patently isn't recursive; but in that case why is
it being declared in a hs-boot file? Generally GHC treats all hs-boot
file types as recursive, because generally it can't see their definitions.
The whole deal about recursive types is an unsatisfactory aspect of GHC.
I'm going to treat this one as fixed for 6.6.1 (via the error message
improvement). But I'll leave it open at low priority for later versions.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/1133>
GHC <http://www.haskell.org/ghc/>
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