#7401: Can't derive instance for Eq when datatype has no constructor, while it
is
trivial do do so.
---------------------------------+------------------------------------------
Reporter: jpbernardy | Owner:
Type: feature request | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.6.1
Keywords: deriving | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: GHC rejects valid program
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
---------------------------------+------------------------------------------
Changes (by igloo):
* difficulty: => Unknown
Old description:
> On a "phantom datatype" D, one gets the message:
>
> Can't make a derived instance of `Eq D':
> `D' must have at least one data constructor
>
> However there is a trivial, correct instance of Eq D:
>
> instance Eq D where
> (==) = undefined
New description:
On a "phantom datatype" D, one gets the message:
{{{
Can't make a derived instance of `Eq D':
`D' must have at least one data constructor
}}}
However there is a trivial, correct instance of Eq D:
{{{
instance Eq D where
(==) = undefined
}}}
--
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7401#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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